warn about breaking inmutability with Proxy

doc/Type/Proxy.pod6

@@ -7,7 +7,9 @@
     class Proxy {}
 
 A Proxy is an object that allows you to execute whenever a value is retrieved
-from a container (C<FETCH>) or when it is set (C<STORE>).
+from a container (C<FETCH>) or when it is set (C<STORE>). Please note that
+C<Proxy> can introduce mutability at places where it would break behaviour,
+e.g. in C<Hash> keys.
 
 To create a container that returns twice of what was stored in it, you do
 something like this: