Rollup merge of rust-lang#46416 - liigo:cfg-macro, r=steveklabnik

doc: macro `cfg!` evaluating at compile-time

Author: GuillaumeGomez

src/libcore/macros.rs

@@ -736,7 +736,7 @@ mod builtin {
     #[cfg(dox)]
     macro_rules! module_path { () => ({ /* compiler built-in */ }) }
 
-    /// Boolean evaluation of configuration flags.
+    /// Boolean evaluation of configuration flags, at compile-time.
     ///
     /// For more information, see the documentation for [`std::cfg!`].
     ///

src/libstd/macros.rs

@@ -631,7 +631,7 @@ pub mod builtin {
     #[macro_export]
     macro_rules! module_path { () => ({ /* compiler built-in */ }) }
 
-    /// Boolean evaluation of configuration flags.
+    /// Boolean evaluation of configuration flags, at compile-time.
     ///
     /// In addition to the `#[cfg]` attribute, this macro is provided to allow
     /// boolean expression evaluation of configuration flags. This frequently