improve explanation as to why we raise NoMethodError on to_ary

features/method_stubs/to_ary.feature

@@ -1,22 +1,24 @@
 Feature: double handling to_ary
 
-  Ruby implicitly sends to_ary to any objects in an Array when the array
-  receives `flatten`:
+  Ruby implicitly sends `to_ary` to all of the objects in an `Array` when the
+  array receives `flatten`:
 
       [obj].flatten # => obj.to_ary
 
+  If `to_ary` raises a `NoMethodError`, Ruby sees that the object can not be coerced
+  into an array and moves on to the next object.
+
   To support this, an RSpec double will raise a NoMethodError when it receives
-  `to_ary`, unless that method is explicitly stubbed.
+  `to_ary` _even if it is set `as_null_object`_, unless `to_ary` is explicitly
+  stubbed.
 
   Scenario: double receiving to_ary
     Given a file named "example.rb" with:
       """
-      describe "a double receiving to_ary" do
+      describe "#to_ary" do
         shared_examples "to_ary" do
-          it "returns nil" do
-            expect do
-              obj.to_ary.should be_nil
-            end.to raise_error(NoMethodError)
+          it "raises a NoMethodError" do
+            expect { obj.to_ary }.to raise_error(NoMethodError)
           end
 
           it "can be overridden with a stub" do
@@ -30,12 +32,12 @@ Feature: double handling to_ary
           end
         end
 
-        context "double as_null_object" do
+        context "sent to a double as_null_object" do
           let(:obj) { double('obj').as_null_object }
           include_examples "to_ary"
         end
 
-        context "double without as_null_object" do
+        context "sent to a double without as_null_object" do
           let(:obj) { double('obj') }
           include_examples "to_ary"
         end