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>

Documentation/networking/netdev-FAQ.rst

@@ -103,7 +103,9 @@ Load the mainline (Linus) page here:
 
 and note the top of the "tags" section.  If it is rc1, it is early in
 the dev cycle.  If it was tagged rc7 a week ago, then a release is
-probably imminent.
+probably imminent. If the most recent tag is a final release tag
+(without an ``-rcN`` suffix) - we are most likely in a merge window
+and ``net-next`` is closed.
 
 How can I tell the status of a patch I've sent?
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