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

Nobody has claimed this yet.

```
// 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.

Start by reviewing the marker-symbol section quoted in the issue and compare its stated values with the behavior seen in GitHub preview. Define the ambiguous terms and the supported input sources, then update the specification and release documentation once the replacement behavior and compatibility expectations are agreed.

Written by the indexing model from the issue text.