Fix documentation of PV Model

[SimonHuette]

I've noticed some flaws in the PV Model documentation that should be fixed. These are:

-Add description of Φ in the calculation of Sunrise Angle
-Swap out Sunrise- and Sunset Angle (sunrise angle should be negative, also matching the code)
-Fix gamma_e in Beam Radiation on Sloped Surface calculation
-Fix Incidence Angle: some terms are printed in italics
-Add description of E_dir,S in Beam Radiation on Sloped Surface calculation
-Remove description of alpha_s in Incidence Angle calculation
-Fix description of alpha_e: Should be "surface azimutz angle" instead of "sun azimuth" (also matching the code)