[llvm][Docs] Explain how to handle excessive formatting changes

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