pgsql: doc: Clarify that OAuth bearer tokens are sent in clear text

doc: Clarify that OAuth bearer tokens are sent in clear text

The bearer token used by OAUTHBEARER authentication is transmitted in
clear text over the network, which makes it vulnerable to sniffing and
similar attacks. This adds a note that SSL connections are recommended
for OAuth authentication.

Backpatch down to v18 where OAuth was introduced.

Author: Zsolt Parragi <zsolt(dot)parragi(at)percona(dot)com>
Co-authored-by: Daniel Gustafsson <daniel(at)yesql(dot)se>
Discussion: https://postgr.es/m/CAN4CZFNy23wWKF+78QG3cfunjJpQWFc7eNP7qcYPB6Sg-NhTQQ@mail.gmail.com
Backpatch-through: 18

Branch
------
REL_18_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/5e41e193f3f401109522eda1dd90d6e2c873bbd7

Modified Files
--------------
doc/src/sgml/client-auth.sgml | 8 ++++++--
doc/src/sgml/protocol.sgml | 10 ++++++++++
2 files changed, 16 insertions(+), 2 deletions(-)