pgsql: doc: Fix PQsslAttribute docs for compression

doc: Fix PQsslAttribute docs for compression

The compression parameter to PQsslAttribute has never returned the
compression method used, it has always returned "on" or "off since
it was added in commit 91fa7b4719ac. Backpatch through v10.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/B9EC60EC-F665-47E8-A221-398C76E382C9@yesql.se
Backpatch-through: v10

Branch
------
REL_11_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/f84e2ed246d20c5bd772f1d1c1fba85faf5a144e

Modified Files
--------------
doc/src/sgml/libpq.sgml | 4 +---
1 file changed, 1 insertion(+), 3 deletions(-)