Clarify `mask` argument in `medial_axis` docstring

Addresses one of the points raised in [this thread](https://groups.google.com/d/msg/scikit-image/XRQy1arQ5E0/GDnYdHrtu4kJ) on the mailing list.
Not related to your changes, but would you also remove the empty line between each parameter? To be consistent with the rest of skimage...