[ci skip] 3.1.9 release

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diff --git a/user_guide_src/source/database/caching.rst b/user_guide_src/source/database/caching.rst
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-######################
-Database Caching Class
-######################
-
-The Database Caching Class permits you to cache your queries as text
-files for reduced database load.
-
-.. important:: This class is initialized automatically by the database
-	driver when caching is enabled. Do NOT load this class manually.
-
-.. important:: Not all query result functions are available when you
-	use caching. Please read this page carefully.
-
-Enabling Caching
-================
-
-Caching is enabled in three steps:
-
--  Create a writable directory on your server where the cache files can
-   be stored.
--  Set the path to your cache folder in your
-   application/config/database.php file.
--  Enable the caching feature, either globally by setting the preference
-   in your application/config/database.php file, or manually as
-   described below.
-
-Once enabled, caching will happen automatically whenever a page is
-loaded that contains database queries.
-
-How Does Caching Work?
-======================
-
-CodeIgniter's query caching system happens dynamically when your pages
-are viewed. When caching is enabled, the first time a web page is
-loaded, the query result object will be serialized and stored in a text
-file on your server. The next time the page is loaded the cache file
-will be used instead of accessing your database. Your database usage can
-effectively be reduced to zero for any pages that have been cached.
-
-Only read-type (SELECT) queries can be cached, since these are the only
-type of queries that produce a result. Write-type (INSERT, UPDATE, etc.)
-queries, since they don't generate a result, will not be cached by the
-system.
-
-Cache files DO NOT expire. Any queries that have been cached will remain
-cached until you delete them. The caching system permits you clear
-caches associated with individual pages, or you can delete the entire
-collection of cache files. Typically you'll want to use the housekeeping
-functions described below to delete cache files after certain events
-take place, like when you've added new information to your database.
-
-Will Caching Improve Your Site's Performance?
-=============================================
-
-Getting a performance gain as a result of caching depends on many
-factors. If you have a highly optimized database under very little load,
-you probably won't see a performance boost. If your database is under
-heavy use you probably will see an improved response, assuming your
-file-system is not overly taxed. Remember that caching simply changes
-how your information is retrieved, shifting it from being a database
-operation to a file-system one.
-
-In some clustered server environments, for example, caching may be
-detrimental since file-system operations are so intense. On single
-servers in shared environments, caching will probably be beneficial.
-Unfortunately there is no single answer to the question of whether you
-should cache your database. It really depends on your situation.
-
-How are Cache Files Stored?
-===========================
-
-CodeIgniter places the result of EACH query into its own cache file.
-Sets of cache files are further organized into sub-folders corresponding
-to your controller functions. To be precise, the sub-folders are named
-identically to the first two segments of your URI (the controller class
-name and function name).
-
-For example, let's say you have a controller called blog with a function
-called comments that contains three queries. The caching system will
-create a cache folder called blog+comments, into which it will write
-three cache files.
-
-If you use dynamic queries that change based on information in your URI
-(when using pagination, for example), each instance of the query will
-produce its own cache file. It's possible, therefore, to end up with
-many times more cache files than you have queries.
-
-Managing your Cache Files
-=========================
-
-Since cache files do not expire, you'll need to build deletion routines
-into your application. For example, let's say you have a blog that
-allows user commenting. Whenever a new comment is submitted you'll want
-to delete the cache files associated with the controller function that
-serves up your comments. You'll find two delete functions described
-below that help you clear data.
-
-Not All Database Functions Work with Caching
-============================================
-
-Lastly, we need to point out that the result object that is cached is a
-simplified version of the full result object. For that reason, some of
-the query result functions are not available for use.
-
-The following functions ARE NOT available when using a cached result
-object:
-
--  num_fields()
--  field_names()
--  field_data()
--  free_result()
-
-Also, the two database resources (result_id and conn_id) are not
-available when caching, since result resources only pertain to run-time
-operations.
-
-******************
-Function Reference
-******************
-
-$this->db->cache_on() / $this->db->cache_off()
-================================================
-
-Manually enables/disables caching. This can be useful if you want to
-keep certain queries from being cached. Example::
-
-	// Turn caching on
-	$this->db->cache_on();
-	$query = $this->db->query("SELECT * FROM mytable");
-	
-	// Turn caching off for this one query
-	$this->db->cache_off();
-	$query = $this->db->query("SELECT * FROM members WHERE member_id = '$current_user'");
-	
-	// Turn caching back on
-	$this->db->cache_on();
-	$query = $this->db->query("SELECT * FROM another_table");
-
-$this->db->cache_delete()
-==========================
-
-Deletes the cache files associated with a particular page. This is
-useful if you need to clear caching after you update your database.
-
-The caching system saves your cache files to folders that correspond to
-the URI of the page you are viewing. For example, if you are viewing a
-page at example.com/index.php/blog/comments, the caching system will put
-all cache files associated with it in a folder called blog+comments. To
-delete those particular cache files you will use::
-
-	$this->db->cache_delete('blog', 'comments');
-
-If you do not use any parameters the current URI will be used when
-determining what should be cleared.
-
-$this->db->cache_delete_all()
-===============================
-
-Clears all existing cache files. Example::
-
-	$this->db->cache_delete_all();
-