Document __has_feature(objc_instancetype).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@139299 91177308-0d34-0410-b5e6-96231b3b80d8

docs/LanguageExtensions.html

@@ -768,6 +768,9 @@ property access via the given method. In all other respects, a method
 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>
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