docs: use doxygen keywords for formatting

Rather than use bold fonts etc, use the correct keywords so doxygen formats
better.

Change-Id: Id9d63f0fc3465665376d7a536c4d6da71998f40e
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/1210
Tested-by: jenkins
Reviewed-by: Øyvind Harboe <oyvindharboe@gmail.com>