add a document for Digest::Base.file.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@10929 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ext/digest/lib/digest.rb

@@ -7,6 +7,10 @@ module Digest
   autoload "SHA512", "digest/sha2"
 
   class Base
+    # creates a digest object and read given file, _name_.
+    # 
+    #  p Digest::SHA256.file("X11R6.8.2-src.tar.bz2").hexdigest
+    #  # => "f02e3c85572dc9ad7cb77c2a638e3be24cc1b5bea9fdbb0b0299c9668475c534"
     def self.file(name)
       digest = self.new
       File.open(name) {|f|