Update docs regarding users' ability to access the _users database. (#630)

Update docs regarding users' ability to access the _users database. (#630)

* Update info about accessing _users

Document now mentions that regular users cannot, by default, access or modify
the _users database. There's also a note about this being changeable in
settings, but that it will be removed in CouchDB 4.0.

* Remove section "Users Public Information"

This section was written for a use case where users read their own docs
from the _users database. Removed, as CouchDB is moving away from supporting
that particular use case.

* Fix formatting

Co-authored-by: Tommi Kärkkäinen <tommi@tommikarkkainen.fi>