Documentation Improvements

Preview:
http://www.stsci.edu/~lbradley/photutils/
This formula appears all over the place ... I have it e.g. here:
https://github.com/gammapy/gammapy/blob/master/gammapy/utils/const.py#L28

We should really provide this to users in `astropy` core (but where to put it) to make it harder for users to screw up.