docs: submitting-patches: clarify the role of LKML

docs: submitting-patches: clarify the role of LKML

The documentation previously stated that LKML should be used as *last
resort*. However, scripts/get_maintainer.pl always suggests it and in a
discussion about changing that[0] it turned out that LKML should in fact
receive all patches.

Update documentation to make it clear that all patches should be sent to
LKML by default, in addition to any subsystem-specific lists.

[0]: https://lore.kernel.org/lkml/19a701a8d5837088aa7d8ba594c228c0e040e747.camel@perches.com/

Signed-off-by: Hannu Hartikainen <hannu@hrtk.in>
Link: https://lore.kernel.org/r/20210707133634.286840-1-hannu@hrtk.in
Signed-off-by: Jonathan Corbet <corbet@lwn.net>