Improve various docstrings

[misi9170]

This PR makes a handful of updates to docstrings throughout FLORIS to respond to minor issues that have been raised.

• Adds details on the serial refine yaw optimizer Documentation: Better documentation of yaw_optimizer_sr.py #1021
• Adds a simple docstring for FlorisModel.get_defaults() FlorisModel.get_defaults() missing docstring #1161
• Updates references to floris.simulation (now floris.core) Documentation still refers to floris.simulation #1105 . In this case, I opted to remove a description of the floris input file structure and referred readers to the docs and the default_inputs.yaml for an example.
• Updates the description docstring in examples/examples_turbine/001_reference_turbines.py, which I forgot to update in Add IEA 22MW reference wind turbine to turbine library #1146 .