docs: netdev: call out the merge window in tag checking

Add the most important case to the question about "where are we
in the cycle" - the case of net-next being closed.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>