[PATCH] doc: Fix pg_combinebackup's description of --sync-method

Hi,

The --sync-method entry on the pg_combinebackup reference page was copied
from pg_basebackup and still says "When the plain format is used" twice,
but pg_combinebackup has no format option: it always writes a plain data
directory. It also says the "backup directory" is synchronized, which is
ambiguous for a tool whose inputs are backup directories; what it syncs is
the output directory (sync_pgdata() on opt.output), always following the
WAL directory and tablespace symbolic links.

The attached patch drops the two references to the plain format and says
"output directory". The text is the same on REL_17_STABLE and
REL_18_STABLE, and the patch applies to both; the docs build and validate.

Regards,
Sai Pranav