Clarify SpriteSolidColor and SpriteCircle docstrings

[pushfoo]

Built & tested locally.

This PR clarifies the SpriteSolidColor and SpriteCircle docstrings by making the following changes:

• Use more precise language in describing the shapes (circle instead of ellipse)
• Succinctly explain the user impact of the texture generation & caching behavior
• Explain the direction of the gradient for SpriteCircle
• Sync the types of arguments and their annotation in the docstrings

[einarf]

I'll just merge this. We can update when we change it to only make white textures.

[pushfoo]

@einarf I think we might want to set color to a keyword arg to get the best of both worlds.

[einarf]

Should not be a problem. Default white then?

[pushfoo]

Yes, for both of them. I'd also like to take consider changing some of the related underlying functionality after I finish my current experiment and reflect on it for a bit.