better documentation for geom_encircle

[bbolker]

I just had a request for the algorithm used by geom_encircle(). This is what I sent; I ought to flesh this out slightly and add it to the docs.

(1) finds the convex hull of the points (2) if desired (based on the 'expand' argument), moves the points defining the convex hull outward away from the centroid (3) constructs an 'xspline' through these points; the details of this (and a literature citation) are given in ?grid::xsplineGrob