pgsql: doc: clarify the use of ssh port forwarding

doc: clarify the use of ssh port forwarding

Reported-by: karimelghazouly@gmail.com

Discussion: /messages/by-id/159854511172.24991.4373145230066586863@wrigleys.postgresql.org

Backpatch-through: 9.5

Branch
------
REL_11_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/fa05c22879225f2fb0b39ee84f915093e7c2d725

Modified Files
--------------
doc/src/sgml/runtime.sgml | 39 ++++++++++++++++++++++-----------------
1 file changed, 22 insertions(+), 17 deletions(-)