Vector source documentation gaps

@swoloszynski is already working on this.

Since Jul 9, 2021.

### Links to related documentation

https://www.mapbox.com/mapbox-gl-js/style-spec/#sources-vector

Some issues:

- [ ] No statement that either "tiles" or "url" are required. (Both marked "optional".)
- [ ] This statement is confusing for me: "All geometric coordinates in vector tiles must be between -1 * extent and (extent * 2) - 1 inclusive." (I don't know what "extent" refers to here, or why this technical detail is important to know here.)
- [ ] No link to [TileJSON spec](https://github.com/mapbox/tilejson-spec).
- [ ] A simple example of a "tiles" value would help a lot, and avoid having to google the TileJSON spec. Eg, `["http://example.com/tiles/mylayer/{z}/{x}/{y}.pbf"]`
- [x] "of the form mapbox://mapid" is confusing to me. Either of these would be better:
* "of the form mapbox://username.tilesetid"
* "of the form mapbox://tilesetid"
- [ ] No links to either "[Slippy map tilenames](https://wiki.openstreetmap.org/wiki/Slippy_map_tilenames) scheme." or "[OSGeo spec scheme](https://wiki.osgeo.org/wiki/Tile_Map_Service_Specification)". Would be much clearer with a very brief description of what each of these entails.