ata: Fix SATA_MOBILE_LPM_POLICY description in Kconfig

Improve the description of the possible default SATA link power
management policies and add the missing description for policy 5.
No functional changes.

Fixes: a5ec5a7b ("ata: ahci: Support state with min power but Partial low power state")
Cc: stable@vger.kernel.org
Signed-off-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Niklas Cassel <cassel@kernel.org>