Documentation: database creation and deletion

Please document a) if the charm creates the postgresql DB itself (it does) and b) if the charm deletes the database when removed (I think it doesn't, but it would be nice to know for sure)

Inspect the charm's database creation and removal lifecycle to verify whether it creates the PostgreSQL database and whether removal deletes it. Document both confirmed behaviors and the relevant caveats; the issue does not name specific files or tests, so begin by locating the charm entry point and lifecycle handlers.

Written by the indexing model from the issue text.