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
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/8075f3f2f8450e71941b418affe29f488277b006

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