Update GC-related comments in dense_map and dense_map_pair
Update GC-related comments in dense_map and dense_map_pair

This removes the nm_register_values and nm_unregister_values calls completely
and adds a note for future reference about what the volatile is doing there.

agisga/nmatrix,thisMagpie/nmatrix,johnmarinelli/nmatrix,wlevine/nmatrix,thisMagpie/nmatrix,wlevine/nmatrix,agisga/nmatrix,janusnic/nmatrix,janusnic/nmatrix,janusnic/nmatrix,wlevine/nmatrix,johnmarinelli/nmatrix,janusnic/nmatrix,thisMagpie/nmatrix,agisga/nmatrix,johnmarinelli/nmatrix,agisga/nmatrix,johnmarinelli/nmatrix,wlevine/nmatrix,thisMagpie/nmatrix