Use & document {escalus_user_db, {module, Mod}} option

The `escalus_user_db` behavior has been streamlined to take one of the three forms:
`{esaclus_user_db, xmpp}`  - for in-band registration
`{escalus_user_db, {module, M}}`  == `{escalus_user_db, {module, M, []}}`
`{escalus_user_db, {module, M, Args}}` - use a module M, that implements `escalus_user_db`, passing Args as the argument to `start/1`