doc: minor clarification in buffer.markdown

Replaced "contents is" with "contents are".
Added a note that initial Buffer contents could contain sensitive data.

PR-URL: https://github.com/nodejs/node/pull/2574


Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>