message: [doc] devtools: mention clang-format dependency
diff: @@ -7,6 +7,8 @@ clang-format-diff.py
  A script to format unified git diffs according to [.clang-format](../../src/.clang-format).
  +Requires `clang-format`, installed e.g. via `brew install clang-format` on macOS.
  +
  For instance, to format the last commit with 0 lines of context,
  the script should be called from the git root folder as follows.
  