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

@@ -30,11 +30,11 @@ use github's pull request workflow to review the contribution, and either ask
 you to make any refinements needed or merge it and make them ourselves. The
 changes will then land on master when we next do a release.
 
-We use `Jenkins <http://matrix.org/jenkins>`_ and 
+We use `Jenkins <http://matrix.org/jenkins>`_ and
 `Travis <https://travis-ci.org/matrix-org/synapse>`_ for continuous
-integration. All pull requests to synapse get automatically tested by Travis; 
-the Jenkins builds require an adminstrator to start them. If your change 
-breaks the build, this will be shown in github, so please keep an eye on the 
+integration. All pull requests to synapse get automatically tested by Travis;
+the Jenkins builds require an adminstrator to start them. If your change
+breaks the build, this will be shown in github, so please keep an eye on the
 pull request for feedback.
 
 Code style
@@ -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
-changelog entry in ``changelog.d/1234.bugfix``, and contain content
-like "The security levels of Florbs are now validated when
-recieved over federation. Contributed by Jane Matrix".
+For example, a fix in PR #1234 would have its changelog entry in
+``changelog.d/1234.bugfix``, and contain content like "The security levels of
+Florbs are now validated when recieved over federation. Contributed by Jane
+Matrix".
 
 Attribution
 ~~~~~~~~~~~
@@ -125,7 +125,7 @@ the contribution or otherwise have the right to contribute it to Matrix::
         personal information I submit with it, including my sign-off) is
         maintained indefinitely and may be redistributed consistent with
         this project or the open source license(s) involved.
-        
+
 If you agree to this for your contribution, then all that's needed is to
 include the line in your commit or pull request comment::