Update LineString constructor documentation

Fix the manual to reflect the new LineString constructor behavior that
accepts a mix of Point instances or coordinates.

Small update to reflect merge of toblerity/Shapely#130. The documentation wasn't synced up and caused some confusion:
http://gis.stackexchange.com/questions/95670/how-to-create-a-shapely-linestring-from-two-points
I should have fixed this in the original PR.