doc: Some improvements in CREATE SUBSCRIPTION ref page

doc: Some improvements in CREATE SUBSCRIPTION ref page

Add link to description of libpq connection strings.  Add link to
explanation of replication access control.  This currently points to the
description of streaming replication access control, which is currently
the same as for logical replication, but that might be refined later.
Also remove plain-text passwords from the examples, to not encourage
that dubious practice.

based on suggestions from Simon Riggs