doc: document unspecified behavior for buf.write* methods

Per https://github.com/nodejs/node/issues/1161, when the
buf.write*() methods are given anything other than what
they expect, indicate that the behavior is unspecified.

Fixes: https://github.com/nodejs/node/issues/1161
PR-URL: https://github.com/nodejs/node/pull/5925


Reviewed-By: Claudio Rodriguez <cjrodr@yahoo.com>