BUG #16206: Documentation for PGDATABASE libpq env var is misleading

The following bug has been logged on the website:

Bug reference: 16206
Logged by: Joachim Jablon
Email address: ewjoachim@gmail.com
PostgreSQL version: 12.1
Operating system: All
Description:

Documentation for PGDATABASE says:

PGDATABASE behaves the same as the dbname connection parameter.

Documentation for dbname says:

The database name. Defaults to be the same as the user name. In certain

contexts, the value is checked for extended formats; see Section 33.1.1 for
more details on those.

But PGDATABASE is not checked for extended formats. This should be
explicitly said in the documentation. I believe a simple sentence like:

PGDATABASE behaves the same as the dbname connection parameter but the

value is not checked for extended format.

might be enough.