Document __has_feature(objc_instancetype).

llvm-svn: 139299
diff --git a/clang/docs/LanguageExtensions.html b/clang/docs/LanguageExtensions.html
index 79c42fd..685544f 100644
--- a/clang/docs/LanguageExtensions.html
+++ b/clang/docs/LanguageExtensions.html
@@ -768,6 +768,9 @@
 with a related result type is treated the same way as method that
 returns <tt>id</tt>.</p>
 
+<p>Use <tt>__has_feature(objc_instancetype)</tt> to determine whether
+the <tt>instancetype</tt> contextual keyword is available.</p>
+
 <!-- ======================================================================= -->
 <h2 id="objc_arc">Automatic reference counting </h2>
 <!-- ======================================================================= -->