[style-spec] update line-gradient doc, add an example expression

[chriswhong]

This PR updates the doc value for the line-gradient property in the style spec.

The previous information was ambiguous and only stated that this property defines a gradient.

The updated information explains that the value must be an expression that makes use of the line-progress input. Especially important is the note that line-progress is the percent distance along the line as measured in the webmercator projected coordinate space (not the length as measured on the wgs84 spheroid).

(This bit of information was critical in debugging some misalignments when attempting to use line-gradient to visualize EV state of charge in the directions api playground)

Also adds an example espression, the same expression used in the line-gradient GL JS example.

[stepankuzmin]

Looks good to me! However, there are multiple unrelated formatting changes. I assume it's some automated tool like Prettier?

[chriswhong]

Thanks for flagging, I fixed it.

[stepankuzmin]

@chriswhong Thanks! Merged