Add note on where-clause side-effects

https://rt.perl.org/Ticket/Display.html?id=130886

Author: zoffixznet

doc/Type/Signature.pod6

@@ -150,6 +150,10 @@ the C<where>-clause inside the sub-signature.
     };
     say one-of-them(c=>42); # OUTPUT: «42␤»
 
+Note: where-clauses that produce side-effects (e.g. printing output,
+pulling from an iterator, or increasing a state variable) are not supported
+and may produce surprising results if used.
+
 =head3 Constraining Optional Arguments
 
 L<Optional arguments|#Optional_and_Mandatory_Parameters> can have constraints,