`marker-symbol` is too ambiguous and/or no longer supported

[varna]

            // OPTIONAL: default ""
            // a symbol to position in the center of this icon
            // if not provided or "", no symbol is overlaid
            // and only the marker is shown
            // Allowed values include
            // - Icon ID
            // - An integer 0 through 9
            // - A lowercase character "a" through "z"
            "marker-symbol": "bus",

• bus renders as an empty marker on GitHub preview, so it probably doesn't work anymore
• what is symbol? Is it JS Symbol? Is it google maps icon name? Is it from Leaflet? This is too ambiguous for a spec.
• what is Icon ID? Where can you get one? Is there a list? Can I add my own? Can I use iconfont id? How do I load an iconfont in a .geojson file? /s
• An integer 0 through 9 and A lowercase character "a" through "z" probably means You can render a character inside of marker if you provide an integer or a lowercase letter as value?
• And there's no mention of the possibility to load icons from URL.

This spec property is horrible and should be properly redefined with a new release.