protocol: Replace '#defines' with 'macros'

The use of # within a description causes the documentation generator
to mistake C syntax with a documentation link.

Remove the # from the documentation, suppressing the warning.

Signed-off-by: Yong Bakos <ybakos@humanoriented.com>
Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>