docs: reporting-issues.rst: CC subsystem and maintainers on regressions

When reporting a regression, users ideally should CC the subsystem and
its maintainers, as that will ensure they get aware of the regression
quickly. And if the culprit is known, they should also CC everyone who
signed if off; the text mentioned the latter in once place already, but
forgot to do so in two other areas where it's relevant.

While fixing this also remind readers to check the mailing list archives
for issues that need to be reported to a bug tracker, as someone might
have reported it by mail only.

All of this got triggered by a recent report where these changes would
have made a difference.

Signed-off-by: Thorsten Leemhuis <linux@leemhuis.info>
Link: https://lore.kernel.org/lkml/dff6badf-58f5-98c8-871c-94d901ac6919@leemhuis.info/
Link: https://lore.kernel.org/lkml/CAJZ5v0hX2StQVttAciHYH-urUH+Hi92z9z2ZbcNgQPt0E2Jpwg@mail.gmail.com/
Link: https://lore.kernel.org/r/dd13f10c30e79e550215e53a8103406daec4e593.1618482489.git.linux@leemhuis.info
Signed-off-by: Jonathan Corbet <corbet@lwn.net>