Fix joint calc power comment

Nithin K struggled with trying to get Joint::calcPower() not to blow up (see [this OpenSim forum thread](https://simtk.org/forums/viewtopic.php?f=91&t=5259#p12977)). Turned out that the state argument must be realized to Acceleration stage first, because the power can come from a prescribed motion constraint where the force can't be known until all the accelerations are calculated. The only real problem was that Joint::calcPower() didn't document its requirements. This PR fixes that.

- Added stage requirement to Joint::calcPower().
- Shortened some comments and code to 80 character width; didn't get them all though.