Add missing connection statuses to docs

The list of connection statuses that PQstatus might return during an
asynchronous connection attempt was outdated:

1. CONNECTION_SETENV is never returned anymore and is only part of the
   enum for backwards compatibility, so remove it from the docs.
2. CONNECTION_CHECK_STANDBY and CONNECTION_GSS_STARTUP were not listed,
   so add them.

CONNECTION_NEEDED and CONNECTION_CHECK_TARGET are not listed in the docs
on purpose, since these are internal states that can never be observed
by a caller of PQstatus.

Author: Jelte Fennema-Nio <postgres@jeltef.nl>
Discussion: https://postgr.es/m/CAGECzQRb21spiiykQ48rzz8w+Hcykz+mB2_hxR65D9Qk6nnw=w@mail.gmail.com

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