clarify that `Box<T>` should only be used when defined *in Rust*
clarify that `Box<T>` should only be used when defined *in Rust*

graydon/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust