docs: netdev: move the patch marking section up

We want people to mark their patches with net and net-next in the subject.
Many miss doing that. Move the FAQ section which points that out up, and
place it after the section which enumerates the trees, that seems like
a pretty logical place for it. Since the two sections are together we
can remove a little bit (not too much) of the repetition.

v2: also remove the text for non-git setups, we want people to use git.

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