Doc'd a few contributing guidelines.

docs/internals/contributing/committing-code.txt

@@ -135,9 +135,10 @@ Django's Git repository:
       specific tasks. Added guidelines of how to use Git, GitHub, and
       how to use pull request together with Trac instead.
 
-  If the patch wasn't a pull request, you should credit the contributors in
-  the commit message: "Thanks A for report, B for the patch and C for the
-  review."
+  Credit the contributors in the commit message: "Thanks A for the report and B
+  for review." Use git's `Co-Authored-By`_ as appropriate.
+
+  .. _Co-Authored-By: https://help.github.com/articles/creating-a-commit-with-multiple-authors/
 
 * For commits to a branch, prefix the commit message with the branch name.
   For example: "[1.4.x] Fixed #xxxxx -- Added support for mind reading."

docs/internals/contributing/writing-documentation.txt

@@ -208,6 +208,9 @@ documentation:
   syntax, it won't be highlighted. Adding ``.. code-block:: python``, for
   example, will force highlighting despite invalid syntax.
 
+* To improve readability, use ``.. admonition:: Descriptive title`` rather than
+  ``.. note::``. Use these boxes sparingly.
+
 * Use these heading styles::
 
     ===