improve comments in psyched_yaml

lib/bundler/psyched_yaml.rb

@@ -4,20 +4,20 @@
 rescue Gem::LoadError
 end if defined?(Gem)
 
-# Psych from stdlib if Syck isn't loaded
+# Psych from stdlib, but only if Syck isn't loaded
 begin
   require 'psych'
 rescue LoadError
 end unless defined?(Syck)
 
-# Psych might NOT EXIST AT ALL, so use the Yaml
-require 'yaml'
+# Psych might NOT EXIST AT ALL, so fall back on yaml
+require 'yaml' unless defined?(YAML)
 
 module Bundler
-  # Now we need a single unified Yaml syntax error
-  if defined?(Psych::SyntaxError)
+  # now we need a unified Yaml syntax error
+  if defined?(Psych::SyntaxError) # Psych
     YamlSyntaxError = Psych::SyntaxError
-  else
+  else # Syck just raises an ArgmuentError
     YamlSyntaxError = ArgumentError
   end
 end