docs: reporting-issues.rst: tone down 'test vanilla mainline' a little

Tell users that reporting bugs with vendor kernels which are only
slightly patched can be okay in some situations, but point out there's a
risk in doing so.

Adjust some related sections to make them compatible and a bit clearer.
At the same time make them less daunting: we want users to report bugs,
even if they can't test vanilla mainline kernel.
Signed-off-by: Thorsten Leemhuis <linux@leemhuis.info>
CC: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/r/652ee20eb36228f5d7ca842299faa4cb472feedb.1616181657.git.linux@leemhuis.infoSigned-off-by: Jonathan Corbet <corbet@lwn.net>