Fix vehicle attribute description in API reference

[hamishwillee]

I think this text from the Vehicle class is wrong:

Most vehicle state is exposed through python attributes (e.g. vehicle.location). Most of these attributes are auto-populated based on the capabilities of the connected autopilot/vehicle.

The attributes reflect current state of the device - location, gps or whatever. They aren't about "capabilities". I suspect that this is really talking about the parameters.

How about change to:

"Most vehicle state is exposed through python attributes (e.g. Vehicle.location), while vehicle capabilities/settings are accessed though named elements of the Vehicle.parameters attribute. Both sets of information are populated from the connected autopilot/vehicle."

@mrpollo Make sense?

[mrpollo]

@geeksville what do you think?