Copter: improve PILOT_SPEED_DN parameter description

[standmit]

Feature request

Is your feature request related to a problem? Please describe.
Special situation when PILOT_SPEED_DN = 0 isn't documented (introduced in #6951). "Oops" happens

Describe the solution you'd like
Special value "0" of PILOT_SPEED_DN parameter should be documented in the parameter description and maybe in AltHold description page.

Describe alternatives you've considered
Alternatively, value "0" should mean "0" but not PILOT_SPEED_UP.

Platform
[ ] All
[ ] AntennaTracker
[ V ] Copter
[ ] Plane
[ ] Rover
[ ] Submarine

Additional context

ardupilot/ArduCopter/Attitude.cpp

Lines 107 to 115 in 0af97b5

	// It will return the PILOT_SPEED_DN value if non zero, otherwise if zero it returns the PILOT_SPEED_UP value.
	uint16_t Copter::get_pilot_speed_dn() const
	{
	if (g2.pilot_speed_dn == 0) {
	return abs(g.pilot_speed_up);
	} else {
	return abs(g2.pilot_speed_dn);
	}
	}

[standmit]

What steps need to be taken to get this updated description into the QGroundControl?

[rmackay9]

@standmit,

I think 4.2 probably needs to go out as the official release. We're almost there, perhaps 2.5 more weeks.

[IamPete1]

Fix is merged and 4.2 is out.