godoc: move overview before API TOC
godoc: move overview before API TOC

Compare:
http://swtch.com/junk/regexp0.html [old]
http://swtch.com/junk/regexp.html [new]

Especially for packages with large APIs, this makes the
overview more promiment, so that it can give the appropriate
context for reading the API list.  This should help significantly
in packages with large APIs, like net, so that the first thing users
see is not a jumble of functions but an introduction to the package.

R=adg, gri, r, kevlar, dsymonds, rogpeppe
CC=golang-dev
http://codereview.appspot.com/5573068

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang