doc,dgram: fix addMembership documentation

Adding membership using `IP_ADD_MEMBERSHIP` with interface address set
to `INADDR_ANY` for `IPv4` or as an index of `0` for `IPv6` leads to
using only one interface selected by the operating system.

Fixes: https://github.com/nodejs/node/issues/1692
PR-URL: https://github.com/nodejs/node/pull/7244


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>