Added OpenAPI document link to usage (#15535)

* Added OpenAPI document link to usage

The OpenAPI document at /api/swagger.v1.json needs an obvious reference.  Sadly, I am English monolingual, so someone else is going to have to do the other languages.  In the mean time, this PR should help anyone looking for the file.

* Update docs/content/doc/developers/api-usage.en-us.md

Co-authored-by: a1012112796 <1012112796@qq.com>

Co-authored-by: techknowlogick <matti@mdranta.net>
Co-authored-by: a1012112796 <1012112796@qq.com>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>

docs/content/doc/developers/api-usage.en-us.md

@@ -79,6 +79,9 @@ API Reference guide is auto-generated by swagger and available on:
 or on
 [gitea demo instance](https://try.gitea.io/api/swagger)
 
+The OpenAPI document is at:
+`https://gitea.your.host/swagger.v1.json`
+
 ## Listing your issued tokens via the API
 
 As mentioned in