[RFC][LangRef][AliasAnalysis] Clarify `noalias` affects only modified objects

I would like to clarify that `noalias` on arguments matches `restrict`
in C99. In particular, objects need to be modified for the `noalias` to
be "effective". We basically apply this reasoning already in various
parts but I want to make it explicit now.

The wording change and the reasons are explained in
  https://reviews.llvm.org/D74935

Please let me know if there are concerns.

Thanks,
  Johannes