Specify reentrancy gurantees of Once::call_once

Author: matklad

src/libstd/sync/once.rs

@@ -178,6 +178,10 @@ impl Once {
     /// happens-before relation between the closure and code executing after the
     /// return).
     ///
+    /// If the given closure recusively invokes `call_once` on the same `Once` 
+    /// instance the exact behavior is not specified, allowed outcomes are
+    /// a panic or a deadlock.
+    ///
     /// # Examples
     ///
     /// ```