Write an explanation and include a picture of each layer type in the style spec docs

[tmcw]

Right now we don't explain layer types in narrative text. They have names and properties and one can infer from those, but we should be more explicit:

[1ec5]

This would be helpful for the Mapbox iOS SDK, since by design, symbols without documentation comments are completely excluded from documentation, as if they don’t exist.

[lucaswoj]

This issue was moved to mapbox/mapbox-gl-js#4123