Keep descriptions of builtin args

[zstadler]

when overridden in custom map args

[zstadler]

I could not find an object that contains the descriptions of the builtin args. They seem to be provided as parameters to arguments.get* calls. For example:

planetiler/planetiler-core/src/main/java/com/onthegomap/planetiler/config/PlanetilerConfig.java

Line 122 in 4c2e187

int minzoom = arguments.getInteger("minzoom", "minimum zoom level", MIN_MINZOOM);