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.

docs/package/gluon-mesh-batman-adv.rst

@@ -197,6 +197,12 @@ assume that there is no multicast router behind this port, meaning
 to only forward multicast to this port if an according multicast
 listener on this link was detected.
 
+Further limitations: IGMP/MLD snooping switches (e.g. "enterprise switches")
+behind the client network of a node (LAN ports) are unsupported. It is
+advised to disable IGMP/MLD snooping on those enterprise switches for now
+or to at least manually mark the port to the Gluon router as a
+"multicast router port".
+
 Alternatively, the filtering of IGMP/MLD reports can be disabled via
 the site.conf (which is not recommended in large meshes though).
 See :ref:`site.conf mesh section <user-site-mesh>` for details.