doc, punycode: soft-deprecation of the punycode module

As discussed and agreed upon by the CTC, the punycode module bundled
in core is soft-deprecated (docs only) for v7 with an eye towards
hard-deprecation in v8 or later.

Also see discussion in https://github.com/nodejs/node/pull/7552

PR-URL: https://github.com/nodejs/node/pull/7941


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>