Changelog should link to PRs

The changelog makes much more sense if it consistently links to PRs, rather
than mostly PRs with the occasional issue #.

CONTRIBUTING.rst

@@ -56,17 +56,17 @@ entry. These are managed by Towncrier
 (https://github.com/hawkowl/towncrier).
 
 To create a changelog entry, make a new file in the ``changelog.d``
-file named in the format of ``issuenumberOrPR.type``. The type can be
+file named in the format of ``PRnumber.type``. The type can be
 one of ``feature``, ``bugfix``, ``removal`` (also used for
 deprecations), or ``misc`` (for internal-only changes). The content of
 the file is your changelog entry, which can contain RestructuredText
 formatting. A note of contributors is welcomed in changelogs for
 non-misc changes (the content of misc changes is not displayed).
 
-For example, a fix for a bug reported in #1234 would have its
+For example, a fix in PR #1234 would have its changelog entry in
-changelog entry in ``changelog.d/1234.bugfix``, and contain content
+``changelog.d/1234.bugfix``, and contain content like "The security levels of
-like "The security levels of Florbs are now validated when
+Florbs are now validated when recieved over federation. Contributed by Jane
-recieved over federation. Contributed by Jane Matrix".
+Matrix".
 
 Attribution
 ~~~~~~~~~~~