update comment in :joint-dof definition #141

Job	Run time
osx	3m 51s
linux (ubuntu:trusty)	3m 45s
doc	3m 53s
linux (ubuntu:xenial)	5m 9s
catkin	5m 26s
linux (ubuntu:bionic)	5m 23s
linux (ubuntu:bionic, PQP)	5m 13s
linux (ubuntu:bionic, BULLET)	5m 23s
linux (ubuntu:focal)	5m 31s
linux (debian:stretch)	5m 10s
linux (osrf/ubuntu_armhf:trusty)	30m 56s
linux (osrf/ubuntu_armhf:xenial)	33m 6s
linux (osrf/ubuntu_arm64:xenial)	34m 30s
linux (arm64v8/ubuntu:bionic)	19m 48s
linux (arm64v8/ubuntu:focal)	1h 30m 12s
linux (osrf/debian_arm64:stretch)	34m 51s
	4h 52m 7s

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