Copter: THR_ACCEL_IMAX param range increased
Copter: reduce throttle to min once landed in RTL

This catches the case where the vehicle lands but the user doesn't
immediately put the throttle to zero.  Before this check it would
continue to attempt to hold it's
Frame_params: Updated 3DR Iris+
Copter: THR_ACCEL_IMAX default to 800
Copter: adjust EKF_CHECK parameter description

Default has been changed to 0.8 for -rc9 so we need to update the
description values so they match