Add altitudeOnly attribute

[hamishwillee]

This adds an attribute in order to indicate a command has an altitude in param 7, but not a lat/lon in param 5/6. It also adds comments to note exactly what the other attributes mean (i.e. a lat/lon in param 3, 4 would not "count").

This allows more precise documentation about issues with using a altitude-only command.

This comes out of MAV call discussion summarised here: mavlink/mavlink#1994 (comment) . There was thought of modifying these to be hasLatLon, and hasAltitude, but decided to reduce churn.

[hamishwillee]

I've decided to close this as "not all that useful". The reason being that while there are commands where we might like to indicate altitudeOnly, they use inconsistent params for the altitude. Given that, there is no way a GCS can reliably use the information. We could revisit, but we'd do so next time we create another altitude command (and suggest we put it in same location as others so this becomes useful.