Minor correction in the doc for non-exclusive pg_start_backup and pg_stop_backup

Hi,

>     These functions cannot be executed during recovery (except
>     <function>pg_is_in_backup</function>, <function>pg_backup_start_time</function>
>     and <function>pg_wal_lsn_diff</function>).

Regarding backup control functions, the func.sgml describes as above.
But non-exclusive pg_start_backup and pg_stop_backup can also be
executed during recovery, and which should be described in the doc.
So I think that the attached patch should be pushed. Thought?

Probably the patch needs to be back-patched to 9.6 where non-exclusive
backup API was added.

Regards,

-- 
Fujii Masao