make Arguments::as_statically_known_str doc(hidden)

(cherry picked from commit cfcc47ea54)

library/core/src/fmt/mod.rs

@@ -743,6 +743,7 @@ impl<'a> Arguments<'a> {
     #[unstable(feature = "fmt_internals", reason = "internal to standard library", issue = "none")]
     #[must_use]
     #[inline]
+    #[doc(hidden)]
     pub fn as_statically_known_str(&self) -> Option<&'static str> {
         let s = self.as_str();
         if core::intrinsics::is_val_statically_known(s.is_some()) { s } else { None }