Improve pvlib.tracking.singleaxis docstring

**Describe the bug**
The docstring for ```pvlib.tracking.singleaxis``` does not explain what the calculated tracker angle is based on.  At the very least it should say that it minimizes AOI and thereby maximizes DNI use.

**Additional context**
In retrospect, I think the ```backtrack``` default should have been ```False``` but that may be more controversial.
