PID: add parameter descriptions

These actually aren't referred to from anywhere but it makes this class more consistent with other PID classes

libraries/PID/PID.cpp

@@ -12,10 +12,27 @@
 extern const AP_HAL::HAL& hal;
 
 const AP_Param::GroupInfo PID::var_info[] = {
+
+    // @Param: P
+    // @DisplayName: PID Proportional Gain
+    // @Description: P Gain which produces an output value that is proportional to the current error value
     AP_GROUPINFO("P",    0, PID, _kp, 0),
+
+    // @Param: I
+    // @DisplayName: PID Integral Gain
+    // @Description: I Gain which produces an output that is proportional to both the magnitude and the duration of the error
     AP_GROUPINFO("I",    1, PID, _ki, 0),
+
+    // @Param: D
+    // @DisplayName: PID Derivative Gain
+    // @Description: D Gain which produces an output that is proportional to the rate of change of the error
     AP_GROUPINFO("D",    2, PID, _kd, 0),
+
+    // @Param: IMAX
+    // @DisplayName: PID Integral Maximum
+    // @Description: The maximum/minimum value that the I term can output
     AP_GROUPINFO("IMAX", 3, PID, _imax, 0),
+
     AP_GROUPEND
 };