Try to make the meaning of 'nocapture' a bit clearer. I

Try to make the meaning of 'nocapture' a bit clearer.  I
commented out the line about simplifylibcalls because I
think this doesn't work in the release (it was fixed in
svn after the release branched).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@65846 91177308-0d34-0410-b5e6-96231b3b80d8