go.tools/cmd/godoc: point at doc.ToHTML documentation for formatting rules
go.tools/cmd/godoc: point at doc.ToHTML documentation for formatting rules

Fixes issue 5429.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/14292043

webfd/go-zh.tools,skyportsystems/go.tools,8limbs/gotools,d0f/go-zh.tools,8limbs/gotools,webfd/go-zh.tools,d0f/go-zh.tools,bryanxu/go-zh.tools,webfd/go-zh.tools,bryanxu/go-zh.tools,d0f/go-zh.tools,d0f/go-zh.tools,webfd/go-zh.tools,bryanxu/go-zh.tools,webfd/go-zh.tools,skyportsystems/go.tools,8limbs/gotools,d0f/go-zh.tools,skyportsystems/go.tools,bryanxu/go-zh.tools,bryanxu/go-zh.tools,skyportsystems/go.tools,skyportsystems/go.tools