Add Clang ReleaseNotes that --autocomplete breaks backward compatibily

Summary:
--autocomplete flag now handles all the flags passed to shell, and this
implementation breaks backward compatibily before Clang 6.0.

Reviewers: teemperor, v.g.vassilev

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D44191

llvm-svn: 326889

clang/docs/ReleaseNotes.rst

@@ -71,6 +71,15 @@ future versions of Clang.
 
 - ...
 
+Modified Compiler Flags
+-----------------------
+
+- Before Clang 7.0, we prepended the "#" character to the --autocomplete argument to
+enable cc1 flags. For example, when the -cc1 or -Xclang flag is in the clang invocation,
+the shell executed clang --autocomplete=#-<flag to be completed>. Clang 7.0 now
+requires the whole invocation including all flags to be passed to the --autocomplete
+like this: clang --autocomplete=-cc1,-xc++,-fsyn.
+
 New Pragmas in Clang
 -----------------------