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: #1692
PR-URL: #7244
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

Author: santigimeno

doc/api/dgram.md

@@ -78,10 +78,11 @@ added: v0.6.9
 * `multicastAddress` {String}
 * `multicastInterface` {String}, Optional
 
-Tells the kernel to join a multicast group at the given `multicastAddress`
-using the `IP_ADD_MEMBERSHIP` socket option. If the `multicastInterface`
-argument is not specified, the operating system will try to add membership to
-all valid networking interfaces.
+Tells the kernel to join a multicast group at the given `multicastAddress` and
+`multicastInterface` using the `IP_ADD_MEMBERSHIP` socket option. If the
+`multicastInterface` argument is not specified, the operating system will choose
+one interface and will add membership to it. To add membership to every
+available interface, call `addMembership` multiple times, once per interface.
 
 ### socket.address()