Clarify f_x

[cwhanse]

In some private functions of pvlib.bifacial.infinite_sheds, the parameter f_x is described as

    f_x : numeric
        Fraction of row slant height from the bottom that is shaded from
        direct irradiance.

In other public functions, the description is

    f_x : numeric
        Fraction of row slant height from the bottom that is shaded. [unitless]

For users trying to understand this algorithm, the different definitions can be confusing, because saying just "shaded" could mean shaded from diffuse irradiance by the adjacent row.

Describe the solution you'd like
Use the longer description for all instances of f_x.

[ayeankit]

hey @cwhanse , is this still open?

[cwhanse]

@ayeankit yes, it is open