Stale description for pg_basebackup

Hello.

It seems to me that there's a stale description in the documentation
of pg_basebackup.

https://www.postgresql.org/docs/13/app-pgbasebackup.html

> Note that there are some limitations in taking a backup from a standby:
...
> If you are using -X none, there is no guarantee that all WAL files
> required for the backup are archived at the end of backup.

Actually, pg_basebackup waits for the all required files to be
archived, which is an established behavior by commit
52f8a59dd9(at)PG10(dot) However, the same commit seems to have forgot to
change the doc for pg_basebackup. (The current description is
introduced by 9a4d51077c(at)PG10)

The attached is a proposal to rewrite it as the following.

+ If you are using -X none, pg_basebackup may wait for a long time for
+ all the required WAL files to be archived. In that case, You may need
+ to call pg_switch_wal() on the primary to complete it sooner.

regards.

--
Kyotaro Horiguchi
NTT Open Source Software Center