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_17_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/c88b36d382ebb7925db9f713dcf5ad8ca1bbae71

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