Improve description of PSF axes

[GernotMaier]

Proposal to clarify the meaning of the axes in the PSF histograms (see also issue #174)

[maxnoe]

It's also not mentioned that these columns are lower and upper bin edges respectively. Maybe we should write it like this?

Columns:

* ``ENERG_LO`` -- ndim: 1, unit: TeV
    Lower bin edges of the true energy axis
* ``ENERG_HI`` -- ndim: 1, unit: TeV
    Upper bin edges of the true energy axis 
* ``THETA_LO`` -- ndim: 1, unit: deg
    Lower bin edges of the axis in the angular distance to center of the array pointing direction (see :ref:`coords-fov`)
*  ``THETA_HI`` -- ndim: 1, unit: deg
    Upper bin edges of the axis in the angular distance to center of the array pointing direction (see :ref:`coords-fov`)
* ``RAD_LO``  -- ndim: 1, unit: deg
     Lower bin edges of the axis in angular distance between simulated and reconstructed direction
* ``RAD_HI`` -- ndim: 1, unit: deg
    Upper bin edges of the axis in angular distance between simulated and reconstructed direction
* ``RPSF`` -- ndim: 3, unit: sr^-1
   Matrix of point spread function values in the binning defined in the above columns :math:`dP/d\Omega`, see :ref:`psf-pdf`.