pgsql: doc: Update RFC URLs

doc: Update RFC URLs

Consistently use the IETF HTML links instead of a random mix of
different sites and formats. Correct one RFC number and fix one broken
link.

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/d54285935072175aac1c446e15ec778b08a8fd75

Modified Files
--------------
doc/src/sgml/client-auth.sgml | 2 +-
doc/src/sgml/json.sgml | 2 +-
doc/src/sgml/libpq.sgml | 2 +-
doc/src/sgml/pgcrypto.sgml | 6 +++---
4 files changed, 6 insertions(+), 6 deletions(-)

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