Improves introduction to Submethod with example refs #3063

Author: JJ

doc/Type/Submethod.pod6

@@ -24,6 +24,31 @@ C<submethod> declarator:
         }
     }
 
+Since submethods are not inherited, an interesting use case is precisely
+methods that are going to be called from the I<standard> submethods such as
+C<BUILD> or C<TWEAK>.
+
+=begin code
+class Hero {
+    has @.inventory;
+    has Str $.name;
+    submethod BUILD( :$!name, :@!inventory ) {
+        @!inventory = self.clean-inventory( @!inventory );
+    }
+    submethod clean-inventory( @inventory ) {
+        @!inventory.unique.sort
+    }
+}
+
+my Hero $þor .= new( name => "Þor",
+                     inventory => ( "Mjollnir", "Megingjörð", "Mjollnir" ) );
+say $þor.inventory;
+# OUTPUT: «[Megingjörð Mjollnir]␤»
+=end code
+
+Invoking these methods make sense only in the specific context of the
+submethod it is invoked from.
+
 =head1 Methods
 
 =head2 method gist