netlink: update outdated comment

The function netlink_clear_multicast_users() was removed as unused
in commit 2173f8d953e7 ("netlink: cleanup tap related functions").
Update the comment in netlink_change_ngroups() to remove the stale
reference, replacing it with a general description of the behavior
while preserving the warning.

Assisted-by: unnamed:deepseek-v3.2 coccinelle
Signed-off-by: Kexin Sun <kexinsun@smail.nju.edu.cn>
Link: https://patch.msgid.link/20260311133519.688-1-kexinsun@smail.nju.edu.cn
Signed-off-by: Jakub Kicinski <kuba@kernel.org>