docs: explain where are settings for release files (#17161)

Just a tiny change to save someone some time while reading the docs. I got lost in the angular code and npm packages trying to reverse engineer it  and figure out what setting controlled this file limit: 

![image](https://user-images.githubusercontent.com/7119703/134829716-8f587878-21a3-413b-ba1e-c2bbe9391ad2.png)

Eventually I tracked it down from

https://github.com/go-gitea/gitea/blob/b74a0f9060d5fda341f5554ef529a2006308c6ab/options/locale/locale_en-US.ini#L2776

to

https://github.com/go-gitea/gitea/blob/b74a0f9060d5fda341f5554ef529a2006308c6ab/templates/repo/upload.tmpl#L11

to 

https://github.com/go-gitea/gitea/blob/b74a0f9060d5fda341f5554ef529a2006308c6ab/web_src/js/index.js#L1043 

to 

https://github.com/go-gitea/gitea/blob/b74a0f9060d5fda341f5554ef529a2006308c6ab/web_src/js/features/dropzone.js#L3

to

https://www.npmjs.com/package/dropzone

and then I realized I can't read 😅,  back  to 

https://github.com/go-gitea/gitea/blob/b74a0f9060d5fda341f5554ef529a2006308c6ab/templates/repo/upload.tmpl#L8

and quickly solved it from there!! 

https://github.com/go-gitea/gitea/blob/b74a0f9060d5fda341f5554ef529a2006308c6ab/modules/upload/upload.go#L81


Yes  I know this long path was 90% me failing to read code properly and going on a red herring journey...  but point is we should not have to do this in the first place to know how to  configure this file upload form 🥺 👉 👈

docs/content/doc/advanced/config-cheat-sheet.en-us.md

@@ -115,6 +115,7 @@ Values containing `#` or `;` must be quoted using `` ` `` or `"""`.
 
 - `ALLOWED_TYPES`: **\<empty\>**: Comma-separated list of allowed file extensions (`.zip`), mime types (`text/plain`) or wildcard type (`image/*`, `audio/*`, `video/*`). Empty value or `*/*` allows all types.
 - `DEFAULT_PAGING_NUM`: **10**: The default paging number of releases user interface
+- For settings related to file attachments on releases, see the `attachment` section.
 
 ### Repository - Signing (`repository.signing`)