pr2_*_action: add pictures visualizing data flow to the docs (ros ticket #3672)

[ahendrix]

From Vijay's doc reviews of pr2_head_action, pr2_gripper_action, and single_joint_action:

• A picture showing data flow from controller to action to high-level app would make this a lot clearer.

trac data:

• Owner: sglaser
• Reporter: gerkey
• Reported at: Wed Jan 20 14:24:53 2010
• URL: https://code.ros.org/trac/wg-ros-pkg/ticket/3672