Predicate documentation should go into more detail

[1ec5]

The documentation comment for four MGLStyleLayer subclass’s predicate property doesn’t go into too much detail, relying instead on a link to the style specification’s Filter documentation to provide more details. But the Filter documentation is expressed in a syntax completely foreign to iOS/macOS developers, who must create an NSPredicate instead of a nested array of strings. Though Apple’s NSPredicate documentation is thorough when it comes to expressable operators, there are a lot of expressable operators that implementations can choose not to implement, and indeed we haven’t implemented operators such as LIKE, BEGINSWITH, and blocks. Moreover, the style specification and Supercluster reserve the following keys:

• $id ([core] Add support for $id key to filters #5708)
• $type
• point_count (Add documentation on cluster behavior + syntax mapbox-gl-js#2985)

All these nuances would cause the predicate property documentation to grow unwieldy. To make matters worse, the property is declared redundantly on four individual MGLStyleLayer subclass. We should consolidate them into a single property: #6406. Beyond that, we could add our first appendix on the subject (realm/jazzy#435), giving predicates a little more discoverability.

/cc @friedbunny @frederoni