Document the forward of the -u option (r211756)

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@212969 91177308-0d34-0410-b5e6-96231b3b80d8

docs/ReleaseNotes.rst

@@ -100,6 +100,8 @@ passes via three new flags: `-Rpass`, `-Rpass-missed` and `-Rpass-analysis`.
 These flags take a POSIX regular expression which indicates the name
 of the pass (or passes) that should emit optimization remarks.
 
+The option `-u` is forwarded to the linker on gnutools toolchains.
+
 New Pragmas in Clang
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