tools: bpftool: add cosmetic changes for the manual pages

Make the look-and-feel of the manual pages somewhat closer to other
manual pages, such as the ones from the utilities from iproute2, by
highlighting more keywords.

Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
Acked-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>