clang-tidy doc: add a note for checkers with an autofix

Summary:
Currently, when looking at a checker documentation, we have to go back
to the whole list or look at the sources to figure out if an autofix
is available or not.

Reviewers: alexfh, aaron.ballman

Subscribers: wuzish, Eugene.Zelenko, nemanjai, kbarton, arphaman, Charusso, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D77983