Document the default Net timeout values by @toolmantim [GH fixes #269]

 * lib/net/{ftp,http,pop,smtp}.rb: added documentation for default values.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_0_0@40295 b2dd03c8-39d4-4d8f-98ff-823fe69b080e