add some doc to stub_chain.feature

features/stubs/stub_chain.feature

@@ -1,31 +1,44 @@
 Feature: stub_chain
 
+  The stub_chain method lets you to stub a chain of methods in one statement.
+  Method chains are considered a design smell, but it's not really the method
+  chain that is the problem - it's the dependency chain represented by a chain
+  of messages to different objects:
+
+    foo.get_bar.get_baz
+
+  This is a Law of Demeter violation if get_bar() returns an object other than
+  foo, and get_baz() returns yet another object.
+
+  Fluent interfaces look similar from a caller's perspective, but don't
+  represent a dependency chain (the caller depends only on the object it is
+  calling). Consider this common example from Ruby on Rails:
+
+    Article.recent.by(current_user)
+
+  The recent() and by() methods return the same object, so this is not
+  a Law of Demeter violation.
+
   Scenario: stub a chain of methods
     Given a file named "stub_chain_spec.rb" with:
       """
       describe "stubbing a chain of methods" do
         subject { Object.new }
 
-        context "given symbols as methods" do
-
+        context "given symbols representing methods" do
           it "returns the correct value" do
             subject.stub_chain(:one, :two, :three).and_return(:four)
-
-            subject.one.two.three.should eql(:four)
+            subject.one.two.three.should eq(:four)
           end
-
         end
 
         context "given a string of methods separated by dots" do
-
           it "returns the correct value" do
             subject.stub_chain("one.two.three").and_return(:four)
-
-            subject.one.two.three.should eql(:four)
+            subject.one.two.three.should eq(:four)
           end
-
         end
       end
       """
     When I run "rspec stub_chain_spec.rb"
-    Then the output should contain "2 examples, 0 failures"
+    Then the output should contain "2 examples, 0 failures"