doc: clarify Buffer.indexOf/lastIndexOf edge cases

PR-URL: https://github.com/nodejs/node/pull/10162
Fixes: https://github.com/nodejs/node/issues/9801


Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Stephen Belanger <admin@stephenbelanger.com>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>