pgsql: Docs: fix pg_wal_lsn_diff manual.

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

Branch
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REL_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/9c89c4bd8d00742b569e7b0e9a49babc7da519d5

Modified Files
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doc/src/sgml/func.sgml | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)