Update and reformat documentation for runtime styling

[1ec5]

Rewrote documentation about MGLShape, its subclasses, and MGLFeature to emphasize runtime styling uses over annotation uses. The documentation comment for each class answers the following questions:

• What kind of shape does it represent?
• What real-world concept would I represent using this kind of shape?
• How would I add this shape to a map using the runtime styling API? Which MGLStyleLayer subclasses apply?
• How would I add this shape to the map as an annotation or overlay? Which MGLMapViewDelegate methods apply?
• How does this class handle edge cases?
• What other classes implement similar shapes or are easily confused with this class?
• What does GeoJSON call this kind of shape, if it differs from the class name?

Rewrote documentation about MGLStyleLayer and its subclasses. The documentation comments are nominally based on the documentation in the style specification. However, all the existing layer types’ documentation has been overridden to answer the following questions:

• What does the output look like?
• What kinds of vector tile features would I style with this class?
• What kinds of MGLShapes and MGLFeatures would I style with this class?
• How do I get an existing layer of this type? How do I add a new one to the style?

Generated documentation comments for MGLStyleLayer subclasses now wrap at column 80. References to properties in other classes have gone from “MGLClass’s property property” to “the MGLClass.property property”, because jazzy can apparently autolink the latter syntax.

Updated links to the GeoJSON specification to point to RFC 7946. Removed references to unimplemented features like circle overlays. Corrected the description of MGLBackgroundStyleLayer, which erroneously stated that the identifier must be “background”, whereas that only happens to be true by default for Studio template styles.

Fixes #6144.

/cc @ericrwolfe

[mention-bot]

@1ec5, thanks for your PR! By analyzing this pull request, we identified @incanus, @boundsj and @yhahn to be potential reviewers.

[1ec5]

Note that “getting the last item in the MGLStyle.layers array” becomes “getting the first item in the MGLStyle.layers array” once #7484 lands.

[friedbunny]

Great changes all around!

[1ec5]

Once fill extrusion layers land on the master branch, we’ll need to provide a similar documentation string for the fill-extrusion type.

/cc @boundsj

[1ec5]

Once this change makes it into the master branch, we’ll need to update this documentation string to note that circle style layers can be filled and optionally stroked.