Hint at how to properly close issues with pull requests (opencast#5257)

Mentioning issues in your PR with some magic keywords can automatically
close these issues when the PR is merged. We even hint at this in our
PR-template already.

However, GitHubs implementation of this does not work super well with
our release/branching model. Specifically, it only works if the PR is
merged into the default branch, which is `develop` in our case.

It **can** be made to work in our situation if the magic phrases are
mentioned in commit messages alternatively or in addition to the PR
description, because we regularly forward merge the release branches.

This patch adds a corresponding note about this to the documentation,
and links it in the PR template.

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,7 +1,7 @@
 ### Your pull request should…
 
 * [ ] have a concise title
-* [ ] [close an accompanying issue](https://help.github.com/en/articles/closing-issues-using-keywords) if one exists
+* [ ] [close an accompanying issue](https://docs.opencast.org/develop/developer/#participate/development-process/#automatically-closing-issues-when-a-pr-is-merged) if one exists
 * [ ] [be against the correct branch](https://docs.opencast.org/develop/developer/development-process#acceptance-criteria-for-patches-in-different-versions)
 * [ ] include migration scripts and documentation, if appropriate
 * [ ] pass automated tests

docs/guides/developer/docs/participate/development-process.md

@@ -107,6 +107,33 @@ There are a couple of rules that committers must follow when merging pull reques
     * It is advised to be pragmatic and only do so if necessary.
 
 
+#### Automatically closing issues when a PR is merged
+
+Our pull request template wants you to "close an accompanying issue."
+This can be done as per the [GitHub documentation](https://help.github.com/en/articles/closing-issues-using-keywords)
+by using one of several magic keywords in front of a valid issue number,
+either in the pull request description, or in any of the commit messages
+of the commits you want to merge. For example:
+
+> This PR **fixes #1234**.
+
+A word of caution: due to our [branching model](#git-repository-branching-model)
+this might not always work as expected. GitHub only recognizes
+the magic words when acting on the default branch of the repository,
+which in our case is `develop`. Issues mentioned in descriptions
+of PRs targeting `develop` or in any message of a commit that lands
+in `develop` will be automatically closed.
+
+Thus, if you are submitting a PR **not** targeting `develop`, and you
+want to use this feature, you **have to** mention the magic words
+in a commit message. Tne PR description does not work in this case.
+And even then, the issue will only be closed, once your merged commits
+reach `develop` by our forward merging process.
+
+Mentioning related issues in the PR description **in addition** to
+the commit message(-s) might of course still be useful for reviewers!
+
+
 Git Repository Branching Model
 ------------------------------