Clarify when fix-it hints on warnings are appropriate

Summary:
This is not a change in the rules, it's meant as a clarification about
warnings. Since the recovery from warnings is a no-op, the fix-it hints
on warnings shouldn't change anything. Anything that doesn't just
suppress the warning and changes the meaning of the code (even if it's
for the better) should be on an additional note.

Reviewers: rsmith, aaron.ballman

Reviewed By: aaron.ballman

Subscribers: cfe-commits, thakis

Tags: #clang

Differential Revision: https://reviews.llvm.org/D62470

llvm-svn: 362266