add irradiance.dni() function to Decomposing and combining irradiance…

[tahentx]

… section of api.rst. this seems to be the most reasonable section to reference the function. Issue #686.

• Closes DOC: irradiance.dni() not exposed in API, but also not private #686
• 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.

Brief description of the problem and proposed solution (if not already fully described in the issue linked to above):
I recommend placing the irradiance.dni() function within the 'Decomposing and combining irradiance' section because it seems the most semantically intuitive option given how the sections are structured.

[wholmgren]

Thanks and welcome @tahentx. I agree that's a good place for the function. Can you add a brief note and your name/username to the 0.6.2 whatsnew file?

[tahentx]

@wholmgren @mikofski thank you for the pull and for the welcome. I'm very excited to be a part of the community and hope to contribute a lot more. I'll add a note to the release notes soon.