`std::env::var` should mention `env!` or `option_env!`

### Location

std::env::var

### Summary

`std::env::var` operates at runtime, `env!` and `option_env!` at compile time.
The reverse link from the macro to the function exists already, but `env!` / `option_env!` should also be mentioned in `std::env::var`'s docs.