Document DataObject.draw and rename other members with leading underscore

[jakkdl]

it seems like draw() is the only publicly documented member of DataObject (although only indirectly at https://hypothesis.readthedocs.io/en/latest/data.html#hypothesis.strategies.data). The DataObject class was previously internal but exposed with https://hypothesis.readthedocs.io/en/latest/changes.html#v4-22-3.

Reading the class def further it does hide __signature__ and warns in the docstring not to use it, so perhaps the only way to find out about .conjecture_data is from reading code elsewhere referring to it, but nonetheless it would make reading code that messes with conjecture_data or count a bit clearer.

I don't see why .draw() should be undocumented in the object and the signature hidden, this seems unfriendly to type checkers and tab completion.