Document map_clone known problems #498
Document map_clone known problems #498

(cherry picked from commit ada0d2c54831a904a53ff4106e0ebb6a0f06a687)

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