[X86] Update command line documentation for -mclwb and -mmovbe which I forgot...

[X86] Update command line documentation for -mclwb and -mmovbe which I forgot in my previous commits.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@294420 91177308-0d34-0410-b5e6-96231b3b80d8