AP_GPS: update parameter descriptions

libraries/AP_GPS/AP_GPS.cpp

@@ -147,6 +147,7 @@ const AP_Param::GroupInfo AP_GPS::var_info[] = {
     // @Param: RATE_MS
     // @DisplayName: GPS update rate in milliseconds
     // @Description: Controls how often the GPS should provide a position update. Lowering below 5Hz is not allowed
+    // @Units: milliseconds
     // @Values: 100:10Hz,125:8Hz,200:5Hz
     // @User: Advanced
     AP_GROUPINFO("RATE_MS", 14, AP_GPS, _rate_ms[0], 200),
@@ -154,6 +155,7 @@ const AP_Param::GroupInfo AP_GPS::var_info[] = {
     // @Param: RATE_MS2
     // @DisplayName: GPS 2 update rate in milliseconds
     // @Description: Controls how often the GPS should provide a position update. Lowering below 5Hz is not allowed
+    // @Units: milliseconds
     // @Values: 100:10Hz,125:8Hz,200:5Hz
     // @User: Advanced
     AP_GROUPINFO("RATE_MS2", 15, AP_GPS, _rate_ms[1], 200),