spec: document the hint for make(map) better #19903
Description
What version of Go are you using (go version)?
go version devel +2e108a1be6 Tue Apr 4 11:55:55 2017 -0700 darwin/amd64
In https://go-review.googlesource.com/c/40113/ Rob Pike asked to add documentation about reflect.MakeMapWithSize.