pgsql: doc: Document that ssl_ciphers does not affect TLS 1.3

doc: Document that ssl_ciphers does not affect TLS 1.3

TLS 1.3 uses a different way of specifying ciphers and a different
OpenSSL API.  PostgreSQL currently does not support setting those
ciphers.  For now, just document this.  In the future, support for
this might be added somehow.

Reviewed-by: Jonathan S. Katz <jkatz@postgresql.org>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>

Branch
------
REL_13_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/dbd03482c626e7c5bb92dde983c4b9de6f623253

Modified Files
--------------
doc/src/sgml/config.sgml | 26 ++++++++++++++++----------
1 file changed, 16 insertions(+), 10 deletions(-)