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_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/a3de685013e7f3c0bf624e037a5569c0d5f8f0c2

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