docs: Warn about the 'partially fixes' GitHub bug.

docs/contributing/version-control.md

@@ -158,7 +158,9 @@ automatically catch common mistakes in the commit message itself.
     message](https://help.github.com/en/articles/closing-issues-via-commit-messages)
     so that the issue is automatically closed when your code is merged.
     Zulip's preferred style for this is to have the final paragraph of
-    the commit message read e.g. "Fixes: \#123."
+    the commit message read e.g. "Fixes: \#123.".
+-   Avoid `Partially fixes #1234`; GitHub's regular expressions ignore
+    the "partially" and close the issue. `Fixes part of #1234` is a good alternative.
 -   Any paragraph content in the commit message should be line-wrapped
     to about 68 characters per line, but no more than 70, so that your
     commit message will be reasonably readable in `git log` in a normal

docs/git/using.md

@@ -321,6 +321,11 @@ to the appropriate issue. Once your commit is merged into zulip/master, GitHub
 will automatically close the referenced issue. See [Closing issues via commit
 messages][github-help-closing-issues] for details.
 
+Note in particular that GitHub's regular expressions for this feature
+are sloppy, so phrases like `Partially fixes #1234` will automatically
+close the issue.  Phrases like `Fixes part of #1234` are a good
+alternative.
+
 Make as many commits as you need to to address the issue or implement your feature.
 
 ## Push your commits to GitHub