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