Merge branch 'netdev-docs'

Jakub Kicinski says:

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docs: netdev: document more of our rules

The patch series length limit and reverse xmas tree are not documented.
Add those, and a tl;dr section summarizing how we differ.

v2: improve the series length blurb (Andrew)
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Signed-off-by: David S. Miller <davem@davemloft.net>