DOC: spatial/qhull: fix some documentation issues
ENH: spatial/qhull: allow obtaining the vertices of the convex hull

This is most useful for 2-D, where qhull computes a counterclockwise
ordering, which otherwise requires some clunky postprocessing.
Merge pull request #2458 from pv/convexhull-vertices

ENH: spatial: Direct access to convex hull vertices