default_alloc_error_hook: explain difference to default __rdl_oom in alloc

Author: RalfJung

library/std/src/alloc.rs

@@ -353,6 +353,9 @@ fn default_alloc_error_hook(layout: Layout) {
     if unsafe { __rust_alloc_error_handler_should_panic != 0 } {
         panic!("memory allocation of {} bytes failed", layout.size());
     } else {
+        // This is the default path taken on OOM, and the only path taken on stable with std.
+        // Crucially, it does *not* call any user-defined code, and therefore there are no potential
+        // reentrancy issues. That makes it different from the default `__rdl_oom` defined in alloc.
         rtprintpanic!("memory allocation of {} bytes failed\n", layout.size());
     }
 }