update comment in :joint-dof definition #136

Job	Run time
doc	3m 40s
catkin	7s
osx	44s
linux (ubuntu:trusty)	3m 55s
linux (ubuntu:xenial)	5m 8s
linux (ubuntu:bionic)	5m 25s
linux (ubuntu:bionic, PQP)	5m 14s
linux (ubuntu:bionic, BULLET)	5m 11s
linux (ubuntu:focal)	5m 45s
linux (debian:stretch)	5m 26s
linux (osrf/ubuntu_armhf:trusty)	31m 13s
linux (osrf/ubuntu_armhf:xenial)	32m 54s
linux (osrf/ubuntu_arm64:trusty)	7s
linux (osrf/ubuntu_arm64:xenial)	34m 24s
linux (arm64v8/ubuntu:bionic)	19m 42s
linux (arm64v8/ubuntu:focal)	7s
linux (osrf/debian_arm64:stretch)	34m 53s
	3h 13m 55s

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