Documentation of mod_http_upload external_secret URL format

Is your feature request related to a problem? Please describe.
I want to use mod_http_upload option external_secret with an self-written php script. To be able to enforce a per user quota, I have to somehow get the JID (or a hash of it) out of the PUT URL.

Describe the solution you'd like
I already inspected the URL mod_http_upload with external_secret generates. It seems to have the format <configured_put_url>/<SHA-1(JID)>/<random>/<filename> (jid_in_url and get_url not configured).
As this format is not documentated, I have to treat it as implementation detail which can change on every update without prior warning.

Please document the URL format in https://docs.ejabberd.im/admin/configuration/#mod-http-upload . To leave space for additional information coded into the URL, I suggest to not say the exact position of the filename but state that it is the last component of the path (see #3189).