Invariant.group and mustalias docs fixes

Summary:
Alias analysis would like to know that
invariant.group.barrier returns pointer that mustalias,
but this can't imply that we can replace one pointer with another

Reviewers: dberlin, sanjoy

Subscribers: llvm-commits, chandlerc, hfinkel, nlewycky, amharc

Differential Revision: https://reviews.llvm.org/D31758

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@300033 91177308-0d34-0410-b5e6-96231b3b80d8