docs: Mention file extension for issue_template directory (#20057)

Author: s-hamann

docs/content/doc/usage/issue-pull-request-templates.en-us.md

@@ -85,7 +85,7 @@ Possible directory names for issue templates:
 - `.gitlab/ISSUE_TEMPLATE`
 - `.gitlab/issue_template`
 
-Inside the directory can be multiple issue templates with the form
+Inside the directory can be multiple markdown (`.md`) issue templates of the form
 
 ```md
 ---