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
------
master

Details
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https://git.postgresql.org/pg/commitdiff/bb8a88bbd1c7a9876b040e9eb54a77af192a52e3

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