docs: netdev-FAQ: fix question headers formatting

Join adjacent questions to a single question line. This fixes the
formatting of questions that were not part of the heading.

Also, drop Q: and A: prefixes. We don't need them now that questions and
answers are visually separated.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Link: https://lore.kernel.org/r/f76078ba5547744f2ec178984c32fbc7dcd29a2b.1608454187.git.baruch@tkos.co.il
Signed-off-by: Jakub Kicinski <kuba@kernel.org>