pgsql: doc: Clarify the system value for sslrootcert

doc: Clarify the system value for sslrootcert

The documentation for the special value "system" for sslrootcert could
be misinterpreted to mean the default operating system CA store, which
it may be, but it's defined to be the default CA store of the SSL lib
used.

Backpatch down to v16 where support for the system value was added.

Author: Daniel Gustafsson <daniel@yesql.se>
Reviewed-by: George MacKerron <george@mackerron.co.uk>
Discussion: https://postgr.es/m/B3CBBAA3-6EA3-4AB7-8619-4BBFAB93DDB4@yesql.se
Backpatch-through: 16

Branch
------
REL_16_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/dda1b06035231e3342fcdb695303c9946989eefb

Modified Files
--------------
doc/src/sgml/libpq.sgml   | 2 +-
doc/src/sgml/runtime.sgml | 3 ++-
2 files changed, 3 insertions(+), 2 deletions(-)