Document the Sprockets compressors

Add documentation to Sprockets::NullCompressor and
Sprockets::LazyCompressor. Also, make LazyCompressor#compressor
private, as it isn't part of the public API.

actionpack/lib/sprockets/compressors.rb

@@ -1,21 +1,37 @@
 module Sprockets
-  class NullCompressor
+  # An asset compressor which does nothing.
+  #
+  # This compressor simply returns the asset as-is, without any compression
+  # whatsoever. It is useful in development mode, when compression isn't
+  # needed but using the same asset pipeline as production is desired.
+  class NullCompressor #:nodoc:
     def compress(content)
       content
     end
   end
 
-  class LazyCompressor
+  # An asset compressor which only initializes the underlying compression
+  # engine when needed.
+  #
+  # This postpones the initialization of the compressor until
+  # <code>#compress</code> is called the first time.
+  class LazyCompressor #:nodoc:
+    # Initializes a new LazyCompressor.
+    #
+    # The block should return a compressor when called, i.e. an object
+    # which responds to <code>#compress</code>.
     def initialize(&block)
       @block = block
     end
 
-    def compressor
-      @compressor ||= @block.call || NullCompressor.new
-    end
-
     def compress(content)
       compressor.compress(content)
     end
+
+    private
+
+    def compressor
+      @compressor ||= (@block.call || NullCompressor.new)
+    end
   end
-end
+end