[X86][docs] Add mention of removal of 'mpx' backend feature to the release notes.

I removed the feature from X86.td in ebe5f17f9c396f14742a7c747951b65eaada158e

diff --git a/llvm/docs/ReleaseNotes.rst b/llvm/docs/ReleaseNotes.rst
index 42867a8..a939a4a 100644 (file)
--- a/llvm/docs/ReleaseNotes.rst
+++ b/llvm/docs/ReleaseNotes.rst
@@ -86,6 +86,10 @@ Changes to the X86 Target
 
 During this release ...
 
+* The 'mpx' feature was removed from the backend. It had been removed from clang
+  frontend in 10.0. Mention of the 'mpx' feature in an IR file will print a
+  message to stderr, but IR should still compile.
+
 Changes to the AMDGPU Target
 -----------------------------