doc +@ in signatures

doc/Type/Signature.pod

@@ -246,6 +246,24 @@ Slurpy parameters have special behaviors when combined with some
 L<traits and modifiers|#Parameter_Traits_and_Modifiers>,
 as described below.
 
+=head2 Single Argument Rule Slurpy
+
+The single argument rule allowes to treat arguments to subroutines,
+C<for>-loops and list constructors based on context. Many methods on positional
+types can work with a single arguments the same way then with a list or
+arguments. Using C<+@> as a sigil in a Signature provides syntactic sugar to
+make that task a little easier. Any single argument of a non-positional type
+will be promoted to a list with a single item.
+
+    sub f(+@a){ dd @a };
+    f(1);
+    # OUTPUT«[1]␤»
+    f(1,2,3);
+    # OUTPUT«[1, 2, 3]␤»
+    my @b = <a b c>;
+    f @b;
+    # OUTPUT«["a", "b", "c"]␤»
+
 =head2 Type Captures
 
 Type Captures allow to defer the specification of a type constraint to the time