add/update YARD documentation

lib/rscons/builder.rb

@@ -1,8 +1,18 @@
 module Rscons
+  # Class to hold an object that knows how to build a certain type of file.
   class Builder
+    # Return a set of default variable values for the Environment to use
+    # unless the user overrides any.
+    # @param env [Environment] The Environment.
     def default_variables(env)
       {}
     end
+
+    # Return whether this builder object is capable of producing a given target
+    # file name from a given source file name.
+    # @param target [String] The target file name.
+    # @param source [String, Array] The source file name(s).
+    # @param env [Environment] The Environment.
     def produces?(target, source, env)
       false
     end