BUGFIX - Cleanup failsafe system so it works correctly with Serial RX

systems and Parallel PWM/PPM systems.

Added setting for failsafe_max_usec.  Renamed failsafe_detect_threshold
to failsafe_min_usec.

Failsafe now detects when a PPM/PWM RX isn't sending ANY data out on
CH1-4.  See documentation notes regarding Graupner receivers in
Failsafe.md.

Documented failsafe system.

docs/Configuration.md

@@ -120,13 +120,13 @@ set frsky_inversion = 1
 
 ## CLI command differences from baseflight
 
-### gps_provider / gps_type
-reason: renamed for consistency
+### gps_type
+reason: renamed to `gps_provider` for consistency
 
-### serialrx_provider / serialrx_type
-reason: renamed for consistency
+### serialrx_type
+reason: renamed to `serialrx_provider` for consistency
 
-### rssi_channel / rssi_aux_channel
+### rssi_aux_channel
 reason: improved functionality
 
 Cleanflight supports using any RX channel for rssi.  Baseflight only supports AUX1 to 4.
@@ -135,3 +135,7 @@ In Cleanflight a value of 0 disables the feature, a higher value indicates the c
 
 Example, to use RSSI on AUX1 in Cleanflight set the value to 5, since 5 is the first AUX channel.
 
+### failsafe_detect_threshold
+reason: improved functionality
+
+See failsafe_min_usec and failsafe_max_usec in Failsafe documentation.

docs/Failsafe.md

@@ -0,0 +1,60 @@
+# Failsafe
+
+There are two types of failsafe:
+
+1) receiver based failsafe
+2) flight controller based failsafe
+
+Receiver based failsafe is where you, from your transmitter and receiver, configure channels to output desired signals if your receiver detects signal loss.
+The idea is that you set throttle and other controls so the aircraft descends in a controlled manner.
+
+Flight controller based failsafe is where the flight controller attempts to detect signal loss from your receiver.
+
+It is possible to use both types at the same time and may be desirable.  Flight controller failsafe can even help if your receiver signal wires come loose, get damaged or your receiver malfunctions in a way the receiver itself cannot detect.
+
+## Flight controller failsafe system 
+
+The failsafe system attempts to detect when your receiver looses signal.  It then attempts to prevent your aircraft from flying away uncontrollably.
+
+The failsafe is activated when:
+
+a) no valid channel data from the RX via Serial RX.
+b) the first 4 Parallel PWM/PPM channels do not have valid signals.
+
+There are a few settings for it, as below.
+
+Failsafe delays are configured in 0.1 second steps.
+
+1 step = 0.1sec
+1 second = 10 steps
+
+
+### `failsafe_delay`
+
+Guard time for failsafe activation after signal lost.
+
+### `failsafe_off_delay`
+
+Delay after failsafe activates before motors finally turn off.  If you fly high you may need more time.
+
+### `failsafe_throttle`
+
+Throttle level used for landing.  Specify a value the causes the aircraft to descend at about 1M/sec.
+
+Use standard RX usec values.  See Rx documentation.
+
+### `failsafe_min_usec`
+
+The shortest PWM/PPM pulse considered valid.
+
+Only valid when using Parallel PWM or PPM receivers.
+
+### `failsafe_max_usec`
+
+The longest PWM/PPM pulse considered valid.
+
+Only valid when using Parallel PWM or PPM receivers.
+
+This setting helps catch when your RX stops sending any data when the RX looses signal.
+
+With a Graupner GR-24 configured for PWM output with failsafe on channels 1-4 set to OFF on the receiver then this setting at it's default value will allow failsafe to be activated.