Rollup merge of #138189 - GuillaumeGomez:env-var, r=joshtriplett

Mention `env` and `option_env` macros in `std::env::var` docs

Fixes #138159.

Just like there are mentions in `env!` and `option_env!` docs to `std::env::var`, it'd be nice to have a "mention back" as well.

library/std/src/env.rs

@@ -202,6 +202,9 @@ impl fmt::Debug for VarsOs {
 /// Returns [`VarError::NotUnicode`] if the variable's value is not valid
 /// Unicode. If this is not desired, consider using [`var_os`].
 ///
+/// Use [`env!`] or [`option_env!`] instead if you want to check environment
+/// variables at compile time.
+///
 /// # Examples
 ///
 /// ```