Add takeoff information to altitude and position control mode docs about px4-user_guide

[bys1123]

Wow, so fast, thanks!

[hamishwillee]

@dagar The statement "The vehicle will take off if the stick is raised above 62.5% percent." is true (it is hard coded) but what does this % refer to?

I THINK that it refers to the full range from the throttle stick bottom position. So in effect, to just above the large dead zone in the middle. Would that be correct?

[dagar]

Yes it's the full range starting from the bottom. It makes more sense when the throttle stick is spring loaded and centers itself.

[bys1123]

@hamishwillee I think this should add to doc when you have time.

[hamishwillee]

Done in #433