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 91fa7b4719. 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

doc/src/sgml/libpq.sgml

@@ -2565,9 +2565,7 @@ const char *PQsslAttribute(const PGconn *conn, const char *attribute_name);
          <term><literal>compression</literal></term>
           <listitem>
            <para>
-            If SSL compression is in use, returns the name of the compression
-            algorithm, or "on" if compression is used but the algorithm is
-            not known. If compression is not in use, returns "off".
+            Returns "on" if SSL compression is in use, else it returns "off".
            </para>
           </listitem>
          </varlistentry>