DOC: note that PR titles should use the commit message prefix convention

The commit message prefix table (BUG, ENH, DOC, etc.) is well
documented, but there is no mention that PR titles should follow
the same convention. In practice this is required by the automated
label mapping in .github/pr-prefix-labeler.yml. Add a short note
in the "Asking for your changes to be merged" section so
contributors know up front.

Author: jeffreyksmithjr

doc/source/dev/development_workflow.rst

@@ -269,6 +269,10 @@ function, add a release note to the ``doc/release/upcoming_changes/``
 directory, following the instructions and format in the
 ``doc/release/upcoming_changes/README.rst`` file.
 
+Use the same prefix convention for your pull request title as for commit
+messages (e.g., ``BUG:``, ``ENH:``, ``DOC:``). This enables automated labeling
+of your PR.
+
 
 .. _workflow_PR_timeline: