godoc: Allow examples for methods.

godoc: Allow examples for methods.

An example for a method M() of type T can be written as
func ExampleT_M() { ... }.
To differentiate between multiple examples for one function, type or
method a suffix with a lowercase start may be appended to the name
of the example function, e.g. ExampleFoo_basicUsage.

Fixes #2465.

R=golang-dev, adg, r, rsc, duperray.olivier, r
CC=golang-dev
https://golang.org/cl/5440100