docs: reporting-issues: reduce quoting and assorted fixes

A pile of small fixes:

- don't quote terms like vanilla, mainline, and stable, unless in they
  occur in places where readers new to the kernel might see them for the
  first time

- make people rule out that vendor patches are interfering if they face
  a regression in a stable or longterm kernel they saw in a vendor
  kernel for the first time

- s/bugs/issues/ in a selected spots

- exchange two headlines that got mixed up somehow

- add a few links to some of the steps in the guide

- Greg mentioned sending reports to the stable mailing list is
  sufficient, so remove the "CC stable maintainers" bits

- fix a few typos and mistakes in the text, with a few very small
  improvements along the way

Signed-off-by: Thorsten Leemhuis <linux@leemhuis.info>
Link: https://lore.kernel.org/r/07bca15d8465b8e234537feb8841dd2ff20243bc.1617113469.git.linux@leemhuis.info


Signed-off-by: Jonathan Corbet <corbet@lwn.net>