Explain MPC_COL_PREV_ANG

[hamishwillee]

This attempts to explain MPC_COL_PREV_ANG as introduced (and discussed in) PX4/PX4-Autopilot#12561

@jkflying and @baumanta you'll recognise the information and diagrams :-)

What has changed a bit here is that I believe that the angle MPC_COL_PREV_ANG is used for all movement, not just going backwards. So its tuning affects what happens all the time.

The other thing that I've tried to capture here is that when you go forward the distance calculated is based on the minimum distance from the whole set of included sensors. So if you're travelling along a wall and including a sensor, that is the distance, that gets included, not the empty space in front of you.

Anyway, please check I got it right. This was hard to understand and convey.

[hamishwillee]

Thanks for that last clarifications @baumanta - that made it clear. I have done a further update to tidy and capture this and merged. I think this is all good now. Thanks also @jkflying .