Rover: shorten failsafe timeouts

radio failsafes may trigger within 1sec (previously was 2 seconds)
default failsafe set to 1.5sec (was 5sec)
also minor improvement to FS_TIMEOUT parameter description

APMrover2/Parameters.cpp

@@ -134,10 +134,12 @@ const AP_Param::Info Rover::var_info[] = {
 
     // @Param: FS_TIMEOUT
     // @DisplayName: Failsafe timeout
-    // @Description: How long a failsafe event need to happen for before we trigger the failsafe action
+    // @Description: The time in seconds that a failsafe condition must persist before the failsafe action is triggered
     // @Units: s
+    // @Range: 1 100
+    // @Increment: 0.5
     // @User: Standard
-    GSCALAR(fs_timeout,    "FS_TIMEOUT",     5),
+    GSCALAR(fs_timeout,    "FS_TIMEOUT",     1.5),
 
     // @Param: FS_THR_ENABLE
     // @DisplayName: Throttle Failsafe Enable

APMrover2/radio.cpp

@@ -131,7 +131,7 @@ void Rover::radio_failsafe_check(uint16_t pwm)
     }
 
     bool failed = pwm < static_cast<uint16_t>(g.fs_throttle_value);
-    if (AP_HAL::millis() - failsafe.last_valid_rc_ms > 2000) {
+    if (AP_HAL::millis() - failsafe.last_valid_rc_ms > 1000) {
         failed = true;
     }
     failsafe_trigger(FAILSAFE_EVENT_THROTTLE, failed);