Documentation improvement #335

[birgits]

This PR implements the docstring issues addressed in #335.

[birgits]

I checked all other occurrences of surface_tilt and think the docstring only needs to be changed in the PVSystem class.
I will also add the default values to the docstrings in the next days.

[birgits]

I added the default values in modelchain.py. I looked how default values are added in other libraries such as numpy, pandas, etc. but there doesn't seem to be one common way. I chose the numpy way but can change it if you like. When we agree on how we want the default values to appear in the docstring, I can also add them to the other modules.

[birgits]

I could also change the orientation_strategy default (#290) if you like. Should this be done in a new PR?

[wholmgren]

I don't have any preference on the style. Maybe others can chime in if they care. There are a couple of pvlib functions that already have defaults, and I think they're in the pandas style.

#290 should be done in a separate PR.

[birgits]

I added all default values in pandas style. Just let me know if there is anything you don't agree with.

[wholmgren]

Thanks, @birgits, this is great. Can you add a note and your name to the 0.4.6 whatsnew file?

[wholmgren]

The failing tests are not related to this PR, so I will go ahead and merge.