Improve various @param docs for src/wp-includes/*.

See #30224.


git-svn-id: https://develop.svn.wordpress.org/trunk@30681 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/cache.php

@@ -664,6 +664,9 @@ class WP_Object_Cache {
 	 * @since 3.4.0
 	 *
 	 * @access protected
+	 * @param string $key
+	 * @param string $group
+	 * @return bool
 	 */
 	protected function _exists( $key, $group ) {
 		return isset( $this->cache[ $group ] ) && ( isset( $this->cache[ $group ][ $key ] ) || array_key_exists( $key, $this->cache[ $group ] ) );