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
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REL_14_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/064e1c879dd28255202e94fdf766f4a98f916858

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