[Doc] Add missing docs to Validator#podfile_from_spec

lib/cocoapods/validator.rb

@@ -449,6 +449,14 @@ module Pod
     #
     attr_reader :source_urls
 
+    # @param  [String] platform_name
+    #         the name of the platform, which should be declared
+    #         in the Podfile.
+    #
+    # @param  [String] deployment_target
+    #         the deployment target, which should be declared in
+    #         the Podfile.
+    #
     # @return [Podfile] a podfile that requires the specification on the
     #         current platform.
     #