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: /messages/by-id/17625-fc47c78b7d71b534@postgresql.org

Branch
------
REL_15_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/cae4688ce81b8449aa6e1e7bfa384d53520a81fb

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