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 <[email protected]> Reviewed-By: Colin Ihrig <[email protected]>

Author: santigimeno

doc/api/dgram.md

@@ -75,10 +75,11 @@ socket.on('message', (msg, rinfo) =>{
 *`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()