media: Documentation: v4l: Add section titles for async

Add section titles for async documentation. While the documentation is
mostly fine as-is, it has grown from its original state but remains
without internal structure. Add it now.

Also remove an extra newline.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Tested-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com> # Renesas RZ/G2L SMARC
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>