docs: reporting-issues.rst: reshuffle and improve TLDR

Make the TLDR a bit shorter while improving it at the same time by going
straight to the aspects readers are more interested it. The change makes
the process especially more straight-forward for people that hit a
regression in a stable or longterm kernel. Due to the changes the TLDR
now also matches the step by step guide better.

Signed-off-by: Thorsten Leemhuis <linux@leemhuis.info>
Link: https://lore.kernel.org/r/762ccd7735315d2fdaa79612fccc1f474881118b.1617113469.git.linux@leemhuis.info


[ jc: fixed transposed _` as noted by Thorsten ]
Signed-off-by: Jonathan Corbet <corbet@lwn.net>