[ci skip] DocBlocks for DB drivers' driver classes

Partially fixes issue #1295.
diff --git a/system/database/DB_driver.php b/system/database/DB_driver.php
index 497f8b9..39db161 100644
--- a/system/database/DB_driver.php
+++ b/system/database/DB_driver.php
@@ -306,6 +306,13 @@
 	protected $_reserved_identifiers	= array('*');
 
 	/**
+	 * Identifier escape character
+	 *
+	 * @var	string
+	 */
+	protected $_escape_char = '"';
+
+	/**
 	 * ESCAPE statement string
 	 *
 	 * @var	string
@@ -320,6 +327,13 @@
 	protected $_like_escape_chr = '!';
 
 	/**
+	 * ORDER BY random keyword
+	 *
+	 * @var	string
+	 */
+	protected $_random_keyword = ' RAND()';
+
+	/**
 	 * COUNT string
 	 *
 	 * @used-by	CI_DB_driver::count_all()