Add commentary about the posts_search filter. See #10667.

git-svn-id: https://develop.svn.wordpress.org/trunk@14603 602fd350-edb4-49c9-b593-d223f7449a82

wp-includes/query.php

@@ -1843,6 +1843,8 @@ class WP_Query {
 					$search .= " AND ($wpdb->posts.post_password = '') ";
 			}
 		}
+		
+		// Allow plugins to contextually add/remove/modify the search section of the database query
 		$search = apply_filters_ref_array('posts_search', array( $search, &$this ) );
 
 		// Category stuff