[NFC] Improve doc string to mention that paths in diff are used as-is

Summary:
Add --relative to the suggested git-diff one liner. If the user does not
pass this argument, then git will produce a diff with the path relative
to the repository root. If the user's working directory is not the
repository root, then clang-format will complain that the file is not
found. The --relative argument makes git produce a diff with the files
relative to the working directory.

Add note to doc string to warn users about the fact that filenames
embedded in the diff are used as-is with no attempts to "do what they
mean, not what they say"

Reviewers: djasper, alexfh, efriedma, klimek, thakis

Reviewed By: efriedma

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D79054