Deprecate the 'mysql', 'sqlite', 'mssql' and 'pdo/dblib' drivers

The 'mysql' PHP extension is notorious for it's problems and is deprecated as of PHP 5.5.

The other ones are dropped from PHP 5.3.
diff --git a/user_guide_src/source/changelog.rst b/user_guide_src/source/changelog.rst
index 906c303..5b01873 100644
--- a/user_guide_src/source/changelog.rst
+++ b/user_guide_src/source/changelog.rst
@@ -20,6 +20,7 @@
 
    -  PHP 5.1.6 is no longer supported. CodeIgniter now requires PHP 5.2.4.
    -  Changed filenaming convention (class file names now must be Ucfirst and everything else in lowercase).
+   -  Changed the default database driver to 'mysqli' (the old 'mysql' driver is DEPRECATED).
    -  ``$_SERVER['CI_ENV']`` can now be set to control the ``ENVIRONMENT`` constant.
    -  Added an optional backtrace to php-error template.
    -  Added Android to the list of user agents.
@@ -139,6 +140,7 @@
 
 -  Database
 
+   -  DEPRECATED the 'mysql', 'sqlite', 'mssql' and 'pdo/dblib' (also known as 'pdo/mssql' or 'pdo/sybase') drivers.
    -  Added **dsn** configuration setting for drivers that support DSN strings (PDO, PostgreSQL, Oracle, ODBC, CUBRID).
    -  Added **schema** configuration setting (defaults to *public*) for drivers that might need it (currently used by PostgreSQL and ODBC).
    -  Added subdrivers support (currently only used by PDO).