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?