doc: Mention more variant --name=value of -c name=value for postgres

doc: Mention more variant --name=value of -c name=value for postgres

postgres --name=value and -c name=value are equivalents.  This commit
expands the documentation of libpq's "option" connection parameter and
the server startup sequence for shell interactions to mention both
rather than only -c.

Extracted from a larger patch by the same authors.

Reported-by: Alexey Palazhchenko
Author: David Johnston, Aleksander Alekseev
Reviewed-by: Nathan Bossart, Peter Eisentraut, Álvaro Herrera
Discussion: https://postgr.es/m/CAJ7c6TMkuLiLfrA+EFCPYfhXoMKRxxssB5c86+ibxfaz6+=Sdg@mail.gmail.com