example: l2fwd: add missing doxygen documentation
example: l2fwd: add missing doxygen documentation

Reviewed-by: Mike Holmes <ed44c09f4d7f698f5510adc894a872c73e08c8bd@linaro.org>
Signed-off-by: Bill Fischofer <52f3c909d51cc5d355a68a403df6906b3c1a8f83@linaro.org>

rsalveti/odp,kalray/odp-mppa,ravineet-singh/odp,kalray/odp-mppa,rsalveti/odp,mike-holmes-linaro/odp,nmorey/odp,mike-holmes-linaro/odp,erachmi/odp,ravineet-singh/odp,erachmi/odp,kalray/odp-mppa,rsalveti/odp,nmorey/odp,ravineet-singh/odp,dkrot/odp,erachmi/odp,kalray/odp-mppa,kalray/odp-mppa,dkrot/odp,kalray/odp-mppa,mike-holmes-linaro/odp,kalray/odp-mppa,ravineet-singh/odp,dkrot/odp,nmorey/odp,dkrot/odp,erachmi/odp,rsalveti/odp,mike-holmes-linaro/odp,nmorey/odp,rsalveti/odp