Docs: fix pg_wal_lsn_diff manual.

The manual did not mention whether its return value is (first arg -
second arg) or (second arg - first arg). The order matters because the
return value could have a sign. Fix the manual so that it mentions the
function returns (first arg - second arg).

Patch reviewed by Tom Lane.

Back-patch through v13. Older version's doc format is difficult to add
more description.
Discussion: https://postgr.es/m/flat/20210206.151125.960423226279810864.t-ishii%40sraoss.co.jp

doc/src/sgml/func.sgml

@@ -25102,11 +25102,11 @@ SELECT collation for ('foo' COLLATE "de_DE");
         <indexterm>
          <primary>pg_wal_lsn_diff</primary>
         </indexterm>
-        <function>pg_wal_lsn_diff</function> ( <parameter>lsn</parameter> <type>pg_lsn</type>, <parameter>lsn</parameter> <type>pg_lsn</type> )
+        <function>pg_wal_lsn_diff</function> ( <parameter>lsn1</parameter> <type>pg_lsn</type>, <parameter>lsn2</parameter> <type>pg_lsn</type> )
         <returnvalue>numeric</returnvalue>
        </para>
        <para>
-        Calculates the difference in bytes between two write-ahead log
+        Calculates the difference in bytes (<parameter>lsn1</parameter> - <parameter>lsn2</parameter>) between two write-ahead log
         locations.  This can be used
         with <structname>pg_stat_replication</structname> or some of the
         functions shown in <xref linkend="functions-admin-backup-table"/> to