fs/f2fs: clarify help messages

For BR2_TARGET_ROOTFS_F2FS_COLD_FILES clarify that extension list must
be a coma separated.

For BR2_TARGET_ROOTFS_F2FS_OVERPROVISION clarify that the default
overprovision ratio is autocalculated according to the partition size.

Signed-off-by: Grzegorz Blach <grzegorz@blach.pl>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>