Some docs and to_s testing

somewhat code_climate inspired

lib/ruby/yield_statement.rb

@@ -1,7 +1,14 @@
 module Ruby
 
+  # Send and yield are very very similar, so they have a base class CallStatement
+  #
+  # The YieldStatement really only provides to_s, and has slightly
+  # different constructor. See CallStatement
+  #
   class YieldStatement < CallStatement
 
+    # We give the instance of the yield and auto generated name
+    # Also, a yield is always (for now) on self
     def initialize(arguments)
       super("yield_#{object_id}".to_sym , SelfExpression.new , arguments)
     end