[llvm][Docs] Explain how to handle excessive formatting changes (#126239)

Based on some feedback in Discord about a PR where a reviewer asked the
author to move the formatting changes to a new PR, which appears to
contradict the current form of this document.

I've added an explanation here, before the point where the author would
be committing any of the formatting changes.

There are other ways this can go, for example some projects don't want
the churn of formatting, or you can pre-emptively send a formatting PR,
but I don't think enumerating them all here will help the audience for
this text.

So I've recomended one path that will start them off well, and can
branch off if the reviewers make requests.

Author: DavidSpickett

llvm/docs/Contributing.rst

@@ -73,15 +73,33 @@ recent commit:
 
   % git clang-format HEAD~1
 
-Note that this modifies the files, but doesn't commit them -- you'll likely want
-to run
+.. note::
+  For some patches, formatting them may add changes that obscure the intent of
+  the patch. For example, adding to an enum that was not previously formatted
+  may result in the entire enum being reformatted. This happens because not all
+  of the LLVM Project conforms to LLVM's clang-format style at this time.
+
+  If you think that this might be the case for your changes, or are unsure, we
+  recommend that you add the formatting changes as a **separate commit** within
+  the Pull Request.
+
+  Reviewers may request that this formatting commit be made into a separate Pull
+  Request that will be merged before your actual changes.
+
+  This means that if the formatting changes are the first commit, you will have
+  an easier time doing this. If they are not, that is ok too, but you will have
+  to do a bit more work to separate it out.
+
+Note that ``git clang-format`` modifies the files, but does not commit them --
+you will likely want to run one of the following to add the changes to a commit:
 
 .. code-block:: console
 
+  # To create a new commit.
+  % git commit -a
+  # To add to the most recent commit.
   % git commit --amend -a
 
-in order to update the last commit with all pending changes.
-
 .. note::
   If you don't already have ``clang-format`` or ``git clang-format`` installed
   on your system, the ``clang-format`` binary will be built alongside clang, and