Document platform specific CLI options

Seems `cordova build android – --versionCode=1` is a thing and works: https://stackoverflow.com/a/40274188/252627

https://cordova.apache.org/docs/en/7.x/reference/cordova-cli/index.html#cordova-build-command has:

<platformOpts> To provide platform specific options, you must include them after – separator. Review platform guide docs for more details.

But in the "platform guide docs" I can't find anything.

filmaj suggested: `--` forwards arguments to the underlying build scripts that are present in each platform.

So these should probably investigated, inventoried and documented.