Add known issue of let binding to format_args doc

Author: Stargateur

library/core/src/macros/mod.rs

@@ -868,6 +868,14 @@ pub(crate) mod builtin {
     /// let s = fmt::format(format_args!("hello {}", "world"));
     /// assert_eq!(s, format!("hello {}", "world"));
     /// ```
+    ///
+    /// # Lifetime limitation
+    ///
+    /// Except when no formatting arguments are used,
+    /// the produced `fmt::Arguments` value borrows temporary values,
+    /// which means it can only be used within the same expression
+    /// and cannot be stored for later use.
+    /// This is a known limitation, see [#92698](https://github.com/rust-lang/rust/issues/92698).
     #[stable(feature = "rust1", since = "1.0.0")]
     #[cfg_attr(not(test), rustc_diagnostic_item = "format_args_macro")]
     #[allow_internal_unsafe]