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

b74a0f9060/options/locale/locale_en-US.ini (L2776)

to

b74a0f9060/templates/repo/upload.tmpl (L11)

to 

b74a0f9060/web_src/js/index.js (L1043) 

to 

b74a0f9060/web_src/js/features/dropzone.js (L3)

to

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

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

b74a0f9060/templates/repo/upload.tmpl (L8)

and quickly solved it from there!! 

b74a0f9060/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`)