Improve custom upload implementation docs

[eric-hc]

Discussed in #3925

^{Originally posted by Logosww December 27, 2024}
According to docs, we should implement the useUploadFile hook to customize the uploader.

So when this hook is implemented, how to use it in editor to actually upload file? There is no rest of the whole solution in docs.

[eric-hc]

This was already discussed here.