Add warning to docs in order to avoid template mismatches (#9411)

* DOCS: add mention of swagger api reference

It's(swagger api link) mentioned vaguely in the FAQ but IMHO missing from API usage page.

* Add warning to avoid template mismatches 

Related to #9320

Author: borisovano

docs/content/doc/advanced/customizing-gitea.en-us.md

@@ -65,7 +65,7 @@ Place the png image at the following path: `custom/public/img/avatar_default.png
 ## Customizing Gitea pages
 
 The `custom/templates` folder allows changing every single page of Gitea. Templates
-to override can be found in the [`templates`](https://github.com/go-gitea/gitea/tree/master/templates) directory of Gitea source. Override by
+to override can be found in the [`templates`](https://github.com/go-gitea/gitea/tree/master/templates) directory of Gitea source (Note: the example link is from `master` branch. Make sure to copy templates from same release you are using). Override by
 making a copy of the file under `custom/templates` using a full path structure
 matching source.