Improve some docs for @param. Remove an unneeded $wpdb global import.

See #30799.


git-svn-id: https://develop.svn.wordpress.org/trunk@30979 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/capabilities.php

@@ -550,7 +550,7 @@ class WP_User {
 	 *
 	 * @param string $field The field to query against: 'id', 'slug', 'email' or 'login'
 	 * @param string|int $value The field value
-	 * @return object Raw user object
+	 * @return object|false Raw user object
 	 */
 	public static function get_data_by( $field, $value ) {
 		global $wpdb;