Add documentation to fields (#173)

Author: christophfroehlich

control_msgs/msg/PidState.msg

@@ -1,13 +1,28 @@
 std_msgs/Header header
 builtin_interfaces/Duration timestep
+
+# error = target - state
 float64 error
+# derivative of error
 float64 error_dot
+
+# equals error
 float64 p_error
+# weighted integral of error
 float64 i_error
+# equals derivative of error
 float64 d_error
+
+# proportional gain
 float64 p_term
+# integral gain
 float64 i_term
+# derivative gain
 float64 d_term
+# upper integral clamp.
 float64 i_max
+# lower integral clamp.
 float64 i_min
+
+# output of the PID controller
 float64 output