Losses usage unclear in PVSystem and ModelChain User Guide

[matsuobasho]

Is your feature request related to a problem? Please describe. Issue pertains to UserGuide. There are 2 issues I see:

1. In the Losses section of the PV System User Guide, the current wording doesn't clearly indicate what goes into this argument.

The losses_parameters attribute contains data that may be used with methods that calculate system losses. At present, these methods include only pvlib.pvsystem.PVSystem.pvwatts_losses() and pvlib.pvsystem.pvwatts_losses(), but we hope to add more related functions and methods in the future.

I think the assumption is that the user knows that input into losses_parameters is a dictionary of losses. With the current wording, I go to the link, then run pvwatts_losses(soiling=2, shading=3, snow=0, mismatch=2, wiring=2, connections=0.5, lid=1.5, nameplate_rating=1, age=0, availability=3) which gives me a float. If I then take this float and use it as an argument to losses_parameters, I get an error.

2. The ModelChain UserGuide doesn't say anything about specifying losses and how these losses pertain to the losses specified PVSystem. May be obvious to an experienced user, but not clear to a new user.

i.e. if we specify a losses_parameters in PVSystem but no losses_model in ModelChain does that do anything? Or vice versa?

Describe the solution you'd like For point

1. I would rephrase that part as:

The losses_parameters attribute contains data that may be used with methods that calculate system losses. This argument accepts a dictionary of losses, as per the losses seen here......

It would also be helpful to just include an example of a PVSystem with losses in this section.

2. In ModelChain Include section explaining how these 2 loss arguments interact with each other. Also include an example ModelChain with a losses_model.

Describe alternatives you've considered Searched the docs, but didn't find answer. I do see the System Model example, but believe the UserGuide should have this information.