xfs: Improve CONFIG_XFS_RT Kconfig help

Improve the description of the XFS_RT configuration option to document
that this option is required for zoned block devices.

Signed-off-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Signed-off-by: Carlos Maiolino <cem@kernel.org>