pgsql: docs: clarify intermediate certificate creation instructions

docs: clarify intermediate certificate creation instructions

Specifically, explain the v3_ca openssl specification.

Discussion: /messages/by-id/20200824175653.GA32411@momjian.us

Backpatch-through: 9.5

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/b1ae70b3b4fd54220a0901eaf3dd4c5ca0827108

Modified Files
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doc/src/sgml/runtime.sgml | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)