I don’t know of any single full documentation, but one immensely useful link someone posted is to https://browse.feddit.de/

There you can much more easily search for communities than the native search seems to allow.

EDIT - just found documentation: https://join-lemmy.org/docs/en/users/02-media.html

I’ve not looked through it yet, so can’t comment on how good it is, but might be useful.