Update modelchain.rst

[cwhanse]

modelchain.rst should be updated to describe use of ModelChain with the following new features:

• ModelChain.with_xxxx methods
• ModelChain.run_model_from_poa and ModelChain.run_model_from_effective_irradiance
• Collection of model output into ModelChain.results when Support for PV systems with multiple arrays #1076 is merged
• Use of ModelChain with a PVSystem with multiple Arrays (again when Support for PV systems with multiple arrays #1076 is merged)
• Use of air, module and/or cell temperature with a ModelChain

ModelChain Examples would be valuable.

[cwhanse]

Recommended text to include in a PR for this issue.