doc: document deprecation of util.is* functions

This patch documentes the deprecation of util.is* functions.
As per the deprecation policy dicussion, nodejs/dev-policy/issues/49,
we need to start with documenting the deprecation. So, this is
the first step towards officially removing them.

PR-URL: https://github.com/nodejs/node/pull/2447


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rod Vagg <rod@vagg.org>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>