alloc/foreign: add warning about non-trivial child allocators

alloc/raw/aligned/foreign.hpp

@@ -34,6 +34,11 @@ namespace util::alloc::raw::aligned {
     ///     alignment
     ///   * using a direct alignment child to service the requests
     ///   * then applying the reverse offset to values as we return the values
+    ///
+    /// This approach will explode if a child allocator wants to write to the
+    /// range, so the user is mostly restricted to very simple allocators
+    /// (like 'linear'). This is why we use a view based at nullptr; any
+    /// mistaken access is quite a lot more likely to fault here.
     template <typename ChildT>
     class foreign {
     public: