Mention std::env::var in env!

When searching for how to read an environment variable, I first encountered the `env!` macro. It would have been useful to me if the documentation had included a link to `std::env::var`, which is what I was actually looking for.

library/core/src/macros/mod.rs

@@ -909,7 +909,10 @@ pub(crate) mod builtin {
     /// Inspects an environment variable at compile time.
     ///
     /// This macro will expand to the value of the named environment variable at
-    /// compile time, yielding an expression of type `&'static str`.
+    /// compile time, yielding an expression of type `&'static str`. Use
+    /// [`std::env::var`] instead if you want to read the value at runtime.
+    ///
+    /// [`std::env::var`]: ../std/env/fn.var.html
     ///
     /// If the environment variable is not defined, then a compilation error
     /// will be emitted. To not emit a compile error, use the [`option_env!`]
@@ -950,7 +953,10 @@ pub(crate) mod builtin {
     /// expand into an expression of type `Option<&'static str>` whose value is
     /// `Some` of the value of the environment variable. If the environment
     /// variable is not present, then this will expand to `None`. See
-    /// [`Option<T>`][Option] for more information on this type.
+    /// [`Option<T>`][Option] for more information on this type.  Use
+    /// [`std::env::var`] instead if you want to read the value at runtime.
+    ///
+    /// [`std::env::var`]: ../std/env/fn.var.html
     ///
     /// A compile time error is never emitted when using this macro regardless
     /// of whether the environment variable is present or not.