docs: submitting-patches: Suggest a longer expected time for responses

While some subsystems do typically have very fast turnaround times on
review this is far from standard over the kernel and is likely to set
unrealistic expectations for submitters.  Tell submitters to expect 2-3
weeks instead, this will cover more of the kernel.

Signed-off-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20230913-submitting-patches-delay-v1-1-a2d48c5ca205@kernel.org