[DOC] cmdline: database-related parameters

@xmo-odoo Can you have a look at this?
Exactly.
It's a regex where:
 - `%h` is replaced by the whole hostname the request is made on.
 - `%d` is replaced by the subdomain the request is made on, with the exception of `www` (so domain `odoo.com` and `www.odoo.com` both match the database `odoo`)