Update docs for API for issue and merge request description templates

doc/api/projects.md

@@ -1344,3 +1344,7 @@ Read more in the [Project members](members.md) documentation.
 ## Project badges
 
 Read more in the [Project Badges](project_badges.md) documentation.
+
+## Issue and merge request description templates
+
+The non-default [issue and merge request description templates](../user/project/description_templates.md) are managed inside the project's repository. So you can manage them via the API through the [Repositories API](repositories.md) and the [Repository Files API](repository_files.md).
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