doc: Specify when ssl_prefer_server_ciphers was added

The ssl_prefer_server_ciphers setting is quite important from a
security point of view, so simply stating that older versions
doesn't have it isn't very helpful.  This adds the version when
the GUC was added to help readers.

Backpatch to all supported versions since this setting has been
around since 9.4.

Reviewed-by: Peter Eisentraut <peter@eisentraut.org>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/5D7E0F5E-E620-4D54-8788-66D421AC76F0@yesql.se
Backpatch-through: v12

doc/src/sgml/config.sgml

@@ -1418,11 +1418,11 @@ include_dir 'conf.d'
        </para>
 
        <para>
-        Older PostgreSQL versions do not have this setting and always use the
+        <productname>PostgreSQL</productname> versions before 9.4 do not have
-        client's preferences.  This setting is mainly for backward
+        this setting and always use the client's preferences.  This setting is
-        compatibility with those versions.  Using the server's preferences is
+        mainly for backward compatibility with those versions.  Using the
-        usually better because it is more likely that the server is appropriately
+        server's preferences is usually better because it is more likely that
-        configured.
+        the server is appropriately configured.
        </para>
       </listitem>
      </varlistentry>