Add __FILE_NAME__ to ReleaseNotes. NFC

Added it under C language changes as a nonstandard
extension for the time being.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@362581 91177308-0d34-0410-b5e6-96231b3b80d8

Author: christinaa

docs/ReleaseNotes.rst

@@ -185,9 +185,11 @@ Windows Support
 C Language Changes in Clang
 ---------------------------
 
-- ...
+- ``__FILE_NAME__`` macro has been added as a Clang specific extension supported
+  in all C-family languages. This macro is similar to ``__FILE__`` except it
+  will always provide the last path component when possible.
 
-...
+- ...
 
 C11 Feature Support
 ^^^^^^^^^^^^^^^^^^^