[LangRef] Clarify that inaccessiblememonly functions are allowed noalias returns

Confusion over this point came up in a couple of recent changes (D117180, e20b32ff). Current tone of discussion seems to be that we think inaccessiblememonly was always legal on allocation functions, so this change takes the form of a clarification instead of a change.

Differential Revision: https://reviews.llvm.org/D117571