mapnik-reference defines which parameters can be used for which type of datasource. This information should be used to validate the parameters given for different layers in the MML file. The information could also be used for documentation of Mapnik datasources and their parameters.

Is there a way I could get a slightly detailed documentation about the music format used? My intent is writing a converter which translates Famitracker exports to binary data directly playable by your player.

Famitracker can be configured to use hardware simmilar to the GB so it could be a good way to compose music. You just use the two default square channels plus the noise channel, the config

Macros (and probably variables) should be documentable so that their docs can be shown on hover or documentLink operations on language server protocols.