Add release notes for the removal of the silent include of altivec.h.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@264236 91177308-0d34-0410-b5e6-96231b3b80d8

docs/ReleaseNotes.rst

@@ -79,6 +79,7 @@ TLS is enabled for Cygwin defaults to -femulated-tls.
 
 C Language Changes in Clang
 ---------------------------
+The -faltivec and -maltivec flags no longer silently include altivec.h on Power platforms.
 
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