Add docstrings to yardoc face

lib/puppet/face/yardoc.rb

@@ -1,10 +1,14 @@
 require 'puppet/face'
 
 Puppet::Face.define(:yardoc, '0.0.1') do
+  summary "Generate Puppet documentation with YARD."
 
   action(:yardoc) do
     default
 
+    summary "Generate module documentation."
+    arguments "[manifest_file.pp ...]"
+
     when_invoked do |*args|
       unless Puppet.features.yard?
         raise RuntimeError, "The 'yard' gem must be installed in order to use this face."