vt1211: Document module parameters

Add a description of the module parameters to the documentation of
the vt1211 driver.
Signed-off-by: NJuerg Haefliger <juergh@gmail.com>
Signed-off-by: NJean Delvare <khali@linux-fr.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>