Improved examples for Digest overview and fixed a broken example in
  Digest::HMAC overview. This patch also adds a description of
  Digest::SHA256.bubblebabble to the Digest overview.

  Patched by @stomar [Bug #9027]


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43668 b2dd03c8-39d4-4d8f-98ff-823fe69b080e