Reinstate #backport-pr-title anchor and add explicit backport PR title guidance (#1855)

Co-authored-by: Stan Ulbrych <stan@python.org>

Author: hugovk

core-team/committing.rst

@@ -149,6 +149,8 @@ bug fixes or security fixes. In almost all cases the fixes should first
 originate on ``main`` and then be ported back to older branches.
 
 
+.. _backport-pr-title:
+
 Backporting changes to an older version
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 

getting-started/git-boot-camp.rst

@@ -622,6 +622,9 @@ The format of a correct backport commit message is:
 
     (cherry picked from commit <commit_sha1>)
 
+Here ``gh-XXXXX`` is the GitHub *issue* number, and ``(GH-XXXXX)`` is the
+original *pull request* number.
+
 An example of a bad backport commit message:
 
 .. code-block:: text
@@ -631,6 +634,17 @@ An example of a bad backport commit message:
 
     * Custom message
 
+.. _backport-pr:
+
+When opening the backport PR, its title PR must follow the same format as the
+commit title, beginning with the ``[<branch>]`` prefix and referencing the
+original PR with a ``(GH-XXXXX)`` suffix. For example:
+
+.. code-block:: text
+   :class: good
+
+    [3.15] gh-12345: Fix the spam module (GH-24680)
+
 After the backport PR is opened, ``miss-islington`` will link it to the original
 PR and remove the corresponding backport label.