MarkDef docs contains composite marks in docstring

[joelostblom]

Follow up on vega/altair#3370.

In VL there is a single generic docstring for mark definitions that is used for both MarkDef and CompositeMarkDef. This means that the docs on MarkDef show that it takes the composite mark strings 'boxplot" etc as arguments, but this is not correct. It would be helpful if the docstring could be modified so that MarkDef show only plain marks and CompositeMarkDef show only composite marks.

If this is as simple as deleting the docstring from the generic ref and creating two new separate ones in the two places I linked above, then I'm happy to create the PR.