doc: discourage use of util.inherits

util.inherits breaks the prototype chain. A fix does not seem
useful, since ES6 extends provides language level support for the
same functionality.

This commit starts fasing out mentions of the method.

Fixes: https://github.com/nodejs/node/issues/6512
Fixes: https://github.com/nodejs/node/issues/4179

PR-URL: https://github.com/nodejs/node/pull/6514


Reviewed-By: James Snell <jasnell@gmail.com>
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>