joint_state_broadcaster: improve docs

[christophfroehlich]

To be honest (maybe out of scope), I think having an example on what "has to be used in combination with" would be beneficial. Especially in terms of different scenarios to using map_interface_to_joint_state.

Also, explaining what leaving the lists empty means would be beneficial in my opinion.

Originally posted by @fmauch in #652 (comment)

[christophfroehlich]

Closed with christophfroehlich#4, kudos to @fmauch