Document bare :D at al in parameter lists

wording improvements are very welcome.
Also, I did not find a proper section to document this,
so I made my own. Suggestions for a better place are
also very welcome.

S06-routines.pod

@@ -10,7 +10,7 @@ Synopsis 6: Subroutines
     Created: 21 Mar 2003
 
     Last Modified: 27 Feb 2015
-    Version: 165
+    Version: 166
 
 This document summarizes Apocalypse 6, which covers subroutines and the
 new type system.
@@ -932,6 +932,26 @@ or function dispatch based on whether the supplied C<Capture>'s first
 argument is marked as an invocant.  For ordinary calls this can
 always be determined at compile time, however.
 
+=head2 Parameters with type constraints
+
+Parameters can be constraint to other types than the default simply by
+using the type name in from of the parameter:
+
+    sub double(Numeric $x) { 2 * $x }
+
+If no explicit type constraint is given, it defaults to the type of the
+surrounding package for method invocants, and to C<Any> everywhere else.
+
+A bare C<:D>, C<:U>, C<:T> or C<:_> instead of a type constraint limits the
+default type to definite, undefined, type objects or any object. The default
+still applies, so in
+
+    class Con {
+        method man(:U: :D $x)
+    }
+
+the signature is equivalent to C<(Con:U: Any:D $x)>.
+
 =head2 Longname parameters
 
 A routine marked with C<multi> can mark part of its parameters to