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

diff --git a/doc/src/sgml/func.sgml b/doc/src/sgml/func.sgml
index b7150510aba464645e900c9c5fa06c66f4917d73..1ab31a9056195bbb2611c4de94fc8d4f34118fa7 100644 (file)
--- a/doc/src/sgml/func.sgml
+++ b/doc/src/sgml/func.sgml
@@ -25102,11 +25102,11 @@ SELECT collation for ('foo' COLLATE "de_DE");
         
          pg_wal_lsn_diff
         
-        pg_wal_lsn_diff ( lsn pg_lsn, lsn pg_lsn )
+        pg_wal_lsn_diff ( lsn1 pg_lsn, lsn2 pg_lsn )
         numeric
        
        
-        Calculates the difference in bytes between two write-ahead log
+        Calculates the difference in bytes (lsn1 - lsn2) between two write-ahead log
         locations.  This can be used
         with pg_stat_replication or some of the
         functions shown in  to