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

For AA-based memory-dependency analysis, this makes sense to me. read/read aliasing doesn’t affect anything.

My only questions is: is there any concern that this might confuse instrumentation passes, or similar, which might do something even for read/read cases? Does anyone know of any other use cases that might be affected by this?

-Hal