Clarify reason for expected memsize in spec.

Author: aardvark179

spec/ruby/optional/capi/typed_data_spec.rb

@@ -11,6 +11,9 @@
 
   it "uses the specified memsize function for ObjectSpace.memsize" do
     @s = CApiAllocTypedSpecs.new
+    # The defined memsize function for the type should return 42 as
+    # the size, and this should be added to the size of the object as
+    # known by Ruby.
     ObjectSpace.memsize_of(@s).should > 42
   end
 end