tell that where clauses are executed on not provided optional arguments

Author: gfldex

doc/Type/Signature.pod6

@@ -152,15 +152,13 @@ the C<where>-clause inside the sub-signature.
 
 =head3 Constraining Optional Arguments
 
-L<Optional arguments|Optional_and_Mandatory_Parameters> can have constraints, too.
-Here we want an optional UInt parameter that, if used, must be greater than 5:
+L<Optional arguments|Optional_and_Mandatory_Parameters> can have constraints,
+too. Any C<where> clause on any parameter will be executed, even if it's
+optional and not provided by the caller. In that case you may have to guard
+against undefined values whitin the C<where> clause.
 
     sub f(Int $a, UInt $i? where { !$i.defined or $i > 5 } ) { ... }
 
-The signature may be made a bit simpler (but more obscure for the less expert eye) by using defaults:
-
-    sub f(Int $a, UInt $i? where { !.defined or $_ > 5 } ) { ... }
-
 =head3 Constraining Slurpy Arguments
 
 L<Slurpy arguments|Slurpy_(A.K.A._Variadic)_Parameters> can not have type