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