Add documentation on cluster behavior + syntax

[mollymerp]

Right now the only things I could find on our clustering functionality were in the examples and in the style reference for GeoJSONSource.

We should document things like the point_count property more explicitly.

Also, I was trying to figure out if there's a way to set a minimum for the number of points in a clusterRadius that should be grouped in a cluster (i.e. if you only want clusters for areas w more than 3 points) and I did not have time to fully check.

@mourner do you have any thoughts?

[mourner]

@mollymerp I don't think we need the minClusterPoints option, but it would be possible to add on the supercluster library level. We should definitely improve docs about clustering.

[YunjieLi]

I also find the point_count a little cryptic, like for a single point in a cluster=true source, its point_count is null instead of 0 (like if I need to filter them out).

[stevage]

So, what properties get generated exactly? cluster (boolean), point_count (integer), any others?

(Are there any other examples of properties generated by Mapbox-G? I wonder if they should be $cluster, $point_count etc to reduce the chance of namespace collisions...)

[tuukka]

I'm seeing cluster, cluster_id, point_count, point_count_abbreviated.

[mourner]

Fixed in #7000