[ci skip] DocBlocks for DB drivers' utility classes

Partially fixes issue #1295.
diff --git a/system/database/drivers/odbc/odbc_forge.php b/system/database/drivers/odbc/odbc_forge.php
index fb85c2a..699ce99 100644
--- a/system/database/drivers/odbc/odbc_forge.php
+++ b/system/database/drivers/odbc/odbc_forge.php
@@ -35,8 +35,15 @@
  */
 class CI_DB_odbc_forge extends CI_DB_forge {
 
+	/**
+	 * DROP TABLE statement
+	 *
+	 * @var	string
+	 */
 	protected $_drop_table		= 'DROP TABLE %s';
 
+	// --------------------------------------------------------------------
+
 	/**
 	 * Create Table
 	 *