Update docs to use hyphens when referencing bugs

Because:
* We have enabled GitHub autolinking[1] in the crash ingestion repos (socorro, tecken, eliot and antenna)
* GitHub autolinking doesn't support spaces when pattern matching, so we've added support to rob-bugson (#willkg/rob-bugson#51) to recognize hyphens, and we want to use hyphens when referencing bugs going forward.

This commit:
* Updates our docs re: conventions when referencing bug ID(s) in git commit subjects and GitHub PR titles
  * Specifically, this replicates the diff already applied to the Socorro repo in #mozilla-services/socorro#6498

[1]: https://docs.github.com/en/get-started/writing-on-github/working-with-advanced-formatting/autolinked-references-and-urls#custom-autolinks-to-external-resources

docs/dev.rst

@@ -117,10 +117,14 @@ work to do, note that in the commit.
 Pull requests
 -------------
 
-Pull request summary should indicate the bug the pull request addresses. For
+Pull request summary should indicate the bug the pull request addresses. Use a hyphen between "bug" and the bug ID(s). For
 example::
 
-    bug 1797200: removed frob from tree class
+    bug-nnnnnnn: removed frog from tree class
+
+For multiple bugs fixed within a single pull request, list the bugs out individually. For example::
+   
+   bug-nnnnnnn, bug-nnnnnnn: removed frog from tree class
 
 Pull request descriptions should cover at least some of the following:
 
@@ -134,7 +138,7 @@ After creating a pull request, attach the pull request to the relevant bugs.
 
 We use the `rob-bugson Firefox addon
 <https://addons.mozilla.org/en-US/firefox/addon/rob-bugson/>`_. If the pull
-request has "bug nnnnnnn: ..." in the summary, then rob-bugson will see that
+request has "bug-nnnnnnn: ..." or "bug-nnnnnnn, bug-nnnnnnn: ..." in the summary, then rob-bugson will see that
 and create a "Attach this PR to bug ..." link.
 
 Then ask someone to review the pull request. If you don't know who to ask, look
@@ -171,10 +175,13 @@ Conventions
 Git conventions
 ---------------
 
-First line is a summary of the commit. It should start with::
+First line is a summary of the commit. It should start with the bug number. Use a hyphen between "bug" and the bug ID(s). For example::
+
+   bug-nnnnnnn: summary
 
-  bug nnnnnnn: summary
+For multiple bugs fixed within a single commit, list the bugs out individually. For example::
 
+   bug-nnnnnnn, bug-nnnnnnn: summary
 
 After that, the commit should explain *why* the changes are being made and any
 notes that future readers should know for context or be aware of.