Clarify interaction between :D and optional params

- Include examples and clarify typecheck explosion
- Remove "passed", since that implies[^1] typecheck
  does not apply to optional params
- Reword "undefined", since there's a difference between
    .defined and .DEFINITE

[1] https://stackoverflow.com/questions/48166325/should-constraining-a-perl-6-named-parameter-to-a-definite-value-make-it-a-requi

Author: zoffixznet

doc/Type/Signature.pod6

@@ -204,7 +204,7 @@ part of the L<colon-pair|/type/Pair>.
 =head3 X<Constraining Defined and Undefined Values|
          type constraint,:D;type constraint,:U;type constraint,:_>
 
-Normally, a type constraint only checks whether the value passed is of the
+Normally, a type constraint only checks whether the value of the parameter is of the
 correct type.
 
     sub limit-lines(Str $s, Int $limit) {
@@ -220,9 +220,9 @@ correct type.
     #     (Str:D $: *%_)»
     say limit-lines "a \n b", Int # Always returns the max number of lines
 
-In the code above, we really only want to deal with defined strings. To
-do this, we use the C<:D> type constraint.  This constraint checks the
-value passed is an I<object instance>, in a similar fashion to calling
+In the code above, we really only want to deal with string instances, not
+type objects. To do this, we use the C<:D> type constraint.  This constraint
+checks the value passed is an I<object instance>, in a similar fashion to calling
 the C<.DEFINITE> method on the value:
 
     sub limit-lines(Str:D $s, Int $limit) { };
@@ -234,7 +234,7 @@ the C<.DEFINITE> method on the value:
 This is much better than the way the program failed before, since here the
 reason for failure is clearer.
 
-It's also possible that undefined types are the only ones that make
+It's also possible that type objects are the only ones that make
 sense for a routine to accept. This can be done with the C<:U> type
 constraint, which checks the value passed if it is a I<type object>,
 rather than an object instance. For example, we can turn the
@@ -276,6 +276,17 @@ The L<Classes and Objects|/language/classtut#Starting_with_class>
 document further elaborates on the concepts of instances and type
 objects and discovering them with the C<.DEFINITE> method.
 
+Keep in mind all parameters have values; even optional ones have
+default defaults that are the type object of the constrained type
+for explicit type constraints or an L<Any> type object if no
+explicit type constraint exists. This means that if you use the 
+C<:D> type smiley, you'd need to provide a default value or make
+the parameter required. Otherwise, the default default would be
+a type object, which would fail the definiteness constraint.
+
+    sub divide (Int:D :$a = 2, Int:D :$b!) { say $a/$b }
+    divide :1a, :2b; # OUTPUT: «0.5␤»
+
 =head3 X«Constraining signatures of Callables|Callable (constrain)»
 
 A L<Callable> parameter can be constrained by its signature, by specifying