[clang] [Clang][Docs] Documented sentinel attribute (PR #196088)

[Erich Keane via cfe-commits]

erichkeane wrote:

Github is not letting me submit a comment?  Perhaps I'll paste it here:

Appertainment shouldn't be the start of the topic?  We should probably try to give a summary of its goal as the first sentence.

Also, this sentence reads REALLY roughly, and could probably use another attempt.

So in summary: Thank you for this!  But could you please take another crack at the text here? 

https://github.com/llvm/llvm-project/pull/196088