Fixed documentation issues in timestamping elements

Signed-off-by: Georgios Katsikas <katsikas.gp@gmail.com>
10 jobs from master in 11 minutes and 25 seconds
Status Job ID Name Coverage
  Test
passed #9973
autobatch

00:02:27

passed #9972
batch

00:02:41

passed #9976
dpdk-batch

00:02:10

passed #9977
dpdk-nobatch

00:01:44

passed #9974
ip6

00:02:31

passed #9975
mt

00:02:23

passed #9979
netmap

00:02:13

passed #9980
netmap_nopool

00:02:20

passed #9981
netmap_nopool_nozc

00:02:16

passed #9978
netmap_single

00:01:43