templates: Improve fixCommitWarning.

- Adds an explanation of why linking the PR to the issue is helpful.
- Links to the commit message documentation.

Author: alya

config/templates/fixCommitWarning.md

@@ -1,4 +1,4 @@
-Hello @{author}, it seems like you have referenced #{issues} in your pull request description, but you have not referenced {issuePronoun} in your commit message description(s). When you reference an issue in a commit message, it automatically closes the corresponding issue when the commit is merged.
+Hello @{author}, it seems like you have referenced #{issues} in your pull request description, but you have not referenced {issuePronoun} in your commit message description(s). Referencing an issue in a commit message automatically closes the corresponding issue when the commit is merged, which makes the issue tracker easier to manage.
 
 Please run `git commit --amend` in your command line client to amend your commit message description with `Fixes #{fixIssues}.`.
 
@@ -14,6 +14,6 @@ Date:   Sat Mar 18 13:42:40 2017 -0700
     Fixes #51.
 ```
 
-Thank you for your contributions to Zulip!
+To learn how to write a great commit message, please refer to our [guide](https://zulip.readthedocs.io/en/latest/contributing/version-control.html#commit-messages).
 
 <!-- fixCommitWarning -->