[clang] [clang][ReleaseNotes] document -fstack-clash-protection for Android (PR #223786)

[Nick Desaulniers via cfe-commits]

nickdesaulniers wrote:

oh, wasn't expecting the auto-squash+merge button to _not wait for code review_.  That's not how every other code review system I've ever used has worked!  oops

https://github.com/llvm/llvm-project/pull/223786