Add docs for undocumented attribute beautify

Docs taken from graphviz@ba776ee9 commit message:

Enable beautify feature in sfdp. This tries to draw leaf nodes uniformly on a circle around root node.

Looks like it just applies to graphs as it is agget(g, "beautify").

It's a bool (because of mapBool function) which defaults to false.

I've attached examples, but the beautify behaviour seems to have a bug when viewed in Graphviz web editor: the circle is divided into one too few sectors, resulting in the first and last node overlapping.

Towards #75