mxData.Rd contents and acov

Just to set a reminder, mxData needs more work documenting the parameters and capturing acov functionality.

We mention four types of data, but only three were listed. `acov` now added as the fourth type, but needs filling out.

`thresholds` and `acov` parameters are listed as not implemented yet, but the function is working with them.

PS: Is sscp format on the todo list or not included by design?