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

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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/f01e3ba56fd125ccba65b23a1829fc25b6f3fc65

Modified Files
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doc/src/sgml/config.sgml | 11 ++++++-----
doc/src/sgml/libpq.sgml  |  7 ++++---
2 files changed, 10 insertions(+), 8 deletions(-)