pgsql: Fix bogus behavior of PQsslAttribute(conn, "library").

Fix bogus behavior of PQsslAttribute(conn, "library").

Commit ebc8b7d44 intended to change the behavior of
PQsslAttribute(NULL, "library"), but accidentally also changed
what happens with a non-NULL conn pointer.  Undo that so that
only the intended behavior change happens.  Clarify some
associated documentation.

Per bug #17625 from Heath Lord.  Back-patch to v15.

Discussion: https://postgr.es/m/17625-fc47c78b7d71b534@postgresql.org

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/80a05679d5a400b8f995740c175f8bdb060bb05c

Modified Files
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doc/src/sgml/libpq.sgml                  | 36 +++++++++++++++++++-------------
src/interfaces/libpq/fe-secure-openssl.c | 13 +++++++++---
2 files changed, 32 insertions(+), 17 deletions(-)