Clarifies relationship between does and isa and ~~

This closes #1556. Actually erases the equivalence between isa and ~~,
because it's simply not true.

Author: JJ

doc/Type/Mu.pod6

@@ -48,16 +48,17 @@ invokes the C<.defined> method on the object and returns its result.
     multi method isa(Mu $type     --> Bool:D)
     multi method isa(Str:D $type  --> Bool:D)
 
-Returns C<True> if the invocant is an instance of class C<$type>, a subset type or a derived class (through inheritance) of C<$type>.
+Returns C<True> if the invocant is an instance of class C<$type>, a subset type or a derived class (through inheritance) of C<$type>. Unlike L<C<does>|/routine/does#(Mu)_routine_does>, which includes roles.
 
     my $i = 17;
     say $i.isa("Int");   # OUTPUT: «True␤»
     say $i.isa(Any);     # OUTPUT: «True␤»
+    role Truish {};
+    my $but-true = 0 but Truish;
+    say $but-true.^name;        # OUTPUT: «Int+{Truish}␤»
+    say $but-true.does(Truish); # OUTPUT: «True␤»
+    say $but-true.isa(Truish);  # OUTPUT: «False␤»
 
-A more idiomatic way to do this is to use the smartmatch operator L<~~|/routine/~~> instead.
-
-    my $s = "String";
-    say $s ~~ Str;       # OUTPUT: «True␤»
 
 =head2 routine does
 
@@ -70,6 +71,10 @@ Returns C<True> if and only if the invocant conforms to type C<$type>.
     say $d.does(Any);                 # True    (Date is a subclass of Any)
     say $d.does(DateTime);            # False   (Date is not a subclass of DateTime)
 
+Unlike L<C<isa>|https://docs.perl6.org/routine/isa#(Mu)_routine_isa>, which returns C<True> only for superclasses, C<does> includes both superclasses and roles.
+
+    say $d.isa(Dateish); # OUTPUT: «False␤»
+
 Using the smart match operator L<~~|/routine/~~> is a more idiomatic alternative.
 
     my $d = Date.new('2016-06-03');