Add comment for the only valid usage of guard -> { respond_to? }

Author: eregon

core/process/_fork_spec.rb

@@ -5,6 +5,8 @@
     Process.respond_to?(:_fork).should == Process.respond_to?(:fork)
   end
 
+  # Using respond_to? in a guard here is OK because the correct semantics
+  # are that _fork is implemented if and only if fork is (see above).
   guard_not -> { Process.respond_to?(:fork) } do
     it "raises a NotImplementedError when called" do
       -> { Process._fork }.should raise_error(NotImplementedError)