Expand docstring for aoi_projection.

[adriesse]

• Enhances the solution for limit cos(aoi) in diffuse sky functions #535
• I am familiar with the contributing guidelines.
• Fully tested. Added and/or modified tests to ensure correct behavior for all reasonable inputs. Tests (usually) must pass on the TravisCI and Appveyor testing services.
• Updates entries to docs/sphinx/source/api.rst for API changes.
• Adds description and name entries in the appropriate docs/sphinx/source/whatsnew file for all changes.
• Code quality and style is sufficient. Passes LGTM and SticklerCI checks.
• New code is fully documented. Includes sphinx/numpydoc compliant docstrings and comments in the code where necessary.
• Pull request is nearly complete and ready for detailed review.

[mikofski]

to improve this even slightly more, can you use LaTeX to add the equation?

$$AOI_{projection} = \\cos{ \\beta } \\cos{ \\left( \\text{ solar zenith } \\right) } + \\sin{ \\beta } \\sin{ \\left( \\text{ solar zenith } \\right) } \\cos{ \\left( \\text{ solar azimuth} \\right) - \\left( \\text{ surface azimuth} \\right) }$$

might require some creative thinking about symbols, and some text here or elsewhere to define them. Just an idea

[adriesse]

Next time?

[wholmgren]

Thanks @adriesse.

Ok with me to merge as is.

[cwhanse]

thanks @adriesse