go.tools/go/types: clearer documentation
go.tools/go/types: clearer documentation

(Follow-up of CL feedback.)

R=adonovan
CC=golang-dev
https://codereview.appspot.com/13333047

t-yuki/go.tools.old,t-yuki/go.tools.old,tmc/mirror-go.tools,tmc/mirror-go.tools,t-yuki/go.tools.old,tmc/mirror-go.tools