Fix broken internal sphinx references

[kandersolar]

• [ ] Closes #xxxx
• I am familiar with the contributing guidelines
• [ ] Tests added
• [ ] Updates entries to docs/sphinx/source/api.rst for API changes.
• [ ] Adds description and name entries in the appropriate "what's new" file in docs/sphinx/source/whatsnew for all changes. Includes link to the GitHub Issue with :issue:`num` or this Pull Request with :pull:`num`. Includes contributor name and/or GitHub username (link with :ghuser:`user`).
• [ ] New code is fully documented. Includes numpydoc compliant docstrings, examples, and comments where necessary.
• Pull request is nearly complete and ready for detailed review.
• Maintainer: Appropriate GitHub Labels and Milestone are assigned to the Pull Request and linked Issue.

Sphinx has a nitpicky mode to emit warnings for references to things it doesn't know about. I enabled it for a local build to see what it did and would not recommend it -- the output is several hundreds or thousands of lines of this:

pvsystem.py:docstring of pvlib.pvsystem.sapm_spectral_loss:: WARNING: py:class reference target not found: numeric
pvsystem.py:docstring of pvlib.pvsystem.sapm_spectral_loss:: WARNING: py:class reference target not found: dict-like
pvsystem.py:docstring of pvlib.pvsystem.scale_voltage_current_power:: WARNING: py:class reference target not found: DataFrame
pvsystem.py:docstring of pvlib.pvsystem.scale_voltage_current_power:: WARNING: py:class reference target not found: numeric
pvsystem.py:docstring of pvlib.pvsystem.scale_voltage_current_power:: WARNING: py:class reference target not found: default 1

But it did flag a few actual broken references that I've fixed here. Marked for 0.9 in anticipation of these changes not being controversial :)

[wholmgren]

thanks @kanderso-nrel