add/update YARD documentation

lib/rscons/builders/object.rb

@@ -1,4 +1,6 @@
 module Rscons
+  # A default RScons builder which knows how to produce an object file from
+  # various types of source files.
   class Object < Builder
     def default_variables(env)
       {

lib/rscons/builders/program.rb

@@ -1,4 +1,6 @@
 module Rscons
+  # A default RScons builder that knows how to link object files into an
+  # executable program.
   class Program < Builder
     def default_variables(env)
       {