docs: gluon-mesh-batman-adv: note regarding IGMP/MLD snooping switches

This adds a warning that entprise switches with an IGMP/MLD snooping
feature are not supported yet with IGMP/MLD filtering enabled.

For this to work, firstly the Linux bridge on the Gluon node needs to
support Multicast Router Discovery (RFC4286). But this feature was only
added to the Linux kernel recently, in 5.1.

Secondly, a Gluon node would need to periodically send "Multicast Router
Advertisment" (RFC4286) messages, to "announce" the multicast router
port setting on bridge port bat0.

Thirdly, the IGMP/MLD snooping switches would need to implement
RFC4286.