AP_GPS: Update comment on get_rate_ms()

AP_GPS::get_rate_ms() is only meant to help the GPS backends

libraries/AP_GPS/AP_GPS.cpp

@@ -1068,7 +1068,10 @@ const Vector3f &AP_GPS::get_antenna_offset(uint8_t instance) const
 }
 
 /*
-  return gps update rate in milliseconds
+  returns the desired gps update rate in milliseconds
+  this does not provide any gurantee that the GPS is updating at the requested
+  rate it is simply a helper for use in the backends for determining what rate
+  they should be configuring the GPS to run at
 */
 uint16_t AP_GPS::get_rate_ms(uint8_t instance) const
 {

libraries/AP_GPS/AP_GPS.h

@@ -392,7 +392,10 @@ protected:
     AP_Float _blend_tc;
 
 private:
-    // return gps update rate in milliseconds
+    // returns the desired gps update rate in milliseconds
+    // this does not provide any gurantee that the GPS is updating at the requested
+    // rate it is simply a helper for use in the backends for determining what rate
+    // they should be configuring the GPS to run at
     uint16_t get_rate_ms(uint8_t instance) const;
 
     struct GPS_timing {