go/types: document default unsafe.Sizeof computation better
go/types: document default unsafe.Sizeof computation better

LGTM=adonovan
R=adonovan
CC=golang-codereviews
https://codereview.appspot.com/155150043

bryanxu/go-zh.tools,bryanxu/go-zh.tools,d0f/go-zh.tools,webfd/go-zh.tools,webfd/go-zh.tools,d0f/go-zh.tools,d0f/go-zh.tools,webfd/go-zh.tools,webfd/go-zh.tools,d0f/go-zh.tools,bryanxu/go-zh.tools,d0f/go-zh.tools,webfd/go-zh.tools,bryanxu/go-zh.tools,bryanxu/go-zh.tools