Tri: _min_throttle interpreted as 0~1000 range for throttle_lower flag

limit.throttle_lower flag becomes true when the throttle passed into the
motors lib (which is in the 0 ~ 1000 range) is below _min throttle.
This makes the interpretation of the THR_MIN parameter consistent
between the main code (which uses 0 ~ 1000 range) and the motors lib
(which previously used the RC3_MIN ~ RC3_MAX range).
The remaining problem however is that the output of the motors continues
to use THR_MIN as if it were a pwm.  I don't believe this is a dangerous
problem however.
MotorsMatrix: _min_throttle interpreted as 0 ~ 1000 range for throttle_lower flag

Also trigger throttle_upper flag when throttle in reaches 1000
SingleCopter: set throttle upper and lower flags
CoaxCopter: set throttle upper and lower flags
AC_AttControl: remove some old comments
AC_PosControl: remove completed to-do comments