[clang-format] State where clang-format-diff.py should be run from

At least with git, file paths in a diff will be relative
to the repo root. So if you are in "llvm-project/lldb"
and the diff shows "clang/foo" modified you get:
No such file or directory

From clang-format-diff.py, since clang-format was
asked to read:
llvm-project/lldb/clang/foo

Add a note to the docs to explain this.

(there is `git diff --relative` but that excludes
changes outside of the current dir)

Reviewed By: sylvestre.ledru

Differential Revision: https://reviews.llvm.org/D91799