change whats new API section names

[wholmgren]

I put an API change in the wrong section of the whats new file in #1008. In case it's a more common misreading, I thought I should check how other packages label these sections.

pvlib:

• API Changes with Deprecations
• API Changes

Pandas:

• Backwards incompatible API changes
• Deprecations

Numpy:

• Expired deprecations
• Compatibility notes
• Deprecations
• C API changes
• Changes

xarray:

• Breaking changes
• Deprecations

For pvlib, I think xarray's pattern is most appropriate. I don't like the similarity of pvlib's sections.

[cwhanse]

I'm OK with the xarray pattern for changes to function signatures and module content.

We may want a third category, for changes which cause functions to produce different results, either from fixing bugs or changing algorithms, e.g., filtering a clearsky model for unreasonable values and replacing with NaN.