docs: Document yet another limitation of tx_queue_size
docs: Document yet another limitation of tx_queue_size

https://bugzilla.redhat.com/show_bug.cgi?id=1484234

Turns out, only vhostuser type of interfaces are supported
currently.

Signed-off-by: Michal Privoznik <83d82aaba2eed257f4814b0c239c260c4caaadf0@redhat.com>

crobinso/libvirt,eskultety/libvirt,andreabolognani/libvirt,crobinso/libvirt,zippy2/libvirt,libvirt/libvirt,zippy2/libvirt,nertpinx/libvirt,fabianfreyer/libvirt,eskultety/libvirt,jfehlig/libvirt,fabianfreyer/libvirt,zippy2/libvirt,jardasgit/libvirt,jfehlig/libvirt,nertpinx/libvirt,olafhering/libvirt,jardasgit/libvirt,andreabolognani/libvirt,libvirt/libvirt,jardasgit/libvirt,andreabolognani/libvirt,andreabolognani/libvirt,zippy2/libvirt,olafhering/libvirt,fabianfreyer/libvirt,libvirt/libvirt,nertpinx/libvirt,olafhering/libvirt,crobinso/libvirt,jfehlig/libvirt,eskultety/libvirt,eskultety/libvirt,fabianfreyer/libvirt,libvirt/libvirt,jfehlig/libvirt,nertpinx/libvirt,jardasgit/libvirt,olafhering/libvirt,jardasgit/libvirt,crobinso/libvirt,fabianfreyer/libvirt,andreabolognani/libvirt,nertpinx/libvirt,eskultety/libvirt