std::env::var and std::env::var_os do not document panic on key containing null byte

[lopopolo]

env::remove_var and env::set_var document that they panic when the key contains a null byte, but env::var and env::var_os do not document this behavior.

https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=e43934f459b10424f374f20db846f092