Clarify documentation

[ddudt]

trafficstars

A running list of things to add or clarify in the documentation:

• [ ] Structure/data type of compute data. For most Optimizable objects, thing.compute returns a dictionary of the computed quantities. But for an OptimizableCollection like a CoilSet, it returns a list of dictionaries.
• [x] #1027 should explain that basis is a possible kwarg for compute and that default is rpz for every vector/tensor quantity
• [x] Show uses of some utility functions somewhere in the docs, or make them part of the public API https://github.com/PlasmaControl/DESC/issues/723

[dpanici]

#723

[dpanici]

Closing as the remaining point is mentioned in docstring of the related classes