Document methods that may return StubUserLang instead of Language

Typehinting parameters that take the return value of these methods
with Language is not safe as they may return global $wgLang which
may or may not be instance of Language.

Bug: T278429
Change-Id: Ia5a71e4c39124f4427bd816e6e19207bb371cc6b

includes/GlobalFunctions.php

@@ -1176,7 +1176,7 @@ function wfConfiguredReadOnlyReason() {
  *                    the current user's language (as a fallback for the old parameter
  *                    functionality), or if it is true then use global object
  *                    for the wiki's content language.
- * @return Language
+ * @return Language|StubUserLang
  */
 function wfGetLangObj( $langcode = false ) {
 	# Identify which language to get or create a language object for.

includes/Title.php

@@ -4389,7 +4389,7 @@ class Title implements LinkTarget, PageIdentity, IDBAccessObject {
 	 * e.g. $wgLang (such as special pages, which are in the user language).
 	 *
 	 * @since 1.18
-	 * @return Language
+	 * @return Language|StubUserLang
 	 */
 	public function getPageLanguage() {
 		global $wgLang, $wgLanguageCode;
@@ -4430,7 +4430,7 @@ class Title implements LinkTarget, PageIdentity, IDBAccessObject {
 	 * e.g. $wgLang (such as special pages, which are in the user language).
 	 *
 	 * @since 1.20
-	 * @return Language
+	 * @return Language|StubUserLang
 	 */
 	public function getPageViewLanguage() {
 		global $wgLang;

includes/parser/Parser.php

@@ -1107,7 +1107,7 @@ class Parser {
 	 *
 	 * @since 1.19
 	 *
-	 * @return Language
+	 * @return Language|StubUserLang
 	 */
 	public function getTargetLanguage() {
 		$target = $this->mOptions->getTargetLanguage();