docs: netdev: turn the net-next closed into a Warning

Use the sphinx Warning box to make the net-next being closed
stand out more.

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

Documentation/networking/netdev-FAQ.rst

@@ -70,8 +70,9 @@ relating to vX.Y
 An announcement indicating when ``net-next`` has been closed is usually
 sent to netdev, but knowing the above, you can predict that in advance.
 
-IMPORTANT: Do not send new ``net-next`` content to netdev during the
-period during which ``net-next`` tree is closed.
+.. warning::
+  Do not send new ``net-next`` content to netdev during the
+  period during which ``net-next`` tree is closed.
 
 Shortly after the two weeks have passed (and vX.Y-rc1 is released), the
 tree for ``net-next`` reopens to collect content for the next (vX.Y+1)