Plane: improve the short and long failsafe docs

thanks to Mark Jacobsen for the analysis!

ArduPlane/Parameters.pde

@@ -517,7 +517,7 @@ const AP_Param::Info var_info[] PROGMEM = {
 
     // @Param: FS_SHORT_ACTN
     // @DisplayName: Short failsafe action
-    // @Description: The action to take on a short (FS_SHORT_TIMEOUT) failsafe event in AUTO, GUIDED or LOITER modes. A short failsafe event in stabilization modes will always cause an immediate change to CIRCLE mode. In AUTO mode you can choose whether it will enter CIRCLE mode or continue with the mission. If FS_SHORT_ACTN is 0 then it will continue with the mission, if it is 1 then it will enter CIRCLE mode, and then enter RTL if the failsafe condition persists for FS_LONG_TIMEOUT seconds. If it is set to 2 then the plane will enter FBWA mode with zero throttle and level attitude to glide in.
+    // @Description: The action to take on a short (FS_SHORT_TIMEOUT) failsafe event. A short failsafe even can be triggered either by loss of RC control (see THR_FS_VALUE) or by loss of GCS control (see FS_GCS_ENABL). A short failsafe event in stabilization and manual modes will cause an change to CIRCLE mode if FS_SHORT_ACTN is 0 or 1, and a change to FBWA mode if FS_SHORT_ACTN is 2. In all other modes (including AUTO and GUIDED mode) a short failsafe event will cause no mode change is FS_SHORT_ACTN is set to 0, will cause a change to CIRCLE mode if set to 1 and will change to FBWA mode if set to 2. Please see the documentation for FS_LONG_ACTN for the behaviour after FS_LONG_TIMEOUT seconds of failsafe.
     // @Values: 0:Continue,1:Circle/ReturnToLaunch,2:Glide
     // @User: Standard
     GSCALAR(short_fs_action,        "FS_SHORT_ACTN",  SHORT_FAILSAFE_ACTION),
@@ -533,7 +533,7 @@ const AP_Param::Info var_info[] PROGMEM = {
 
     // @Param: FS_LONG_ACTN
     // @DisplayName: Long failsafe action
-    // @Description: The action to take on a long (FS_LONG_TIMEOUT seconds) failsafe event in AUTO, GUIDED or LOITER modes. A long failsafe event in stabilization modes will always cause an RTL (ReturnToLaunch). In AUTO modes you can choose whether it will RTL or continue with the mission. If FS_LONG_ACTN is 0 then it will continue with the mission, if it is 1 then it will enter RTL mode. Note that if FS_SHORT_ACTN is 1, then the aircraft will enter CIRCLE mode after FS_SHORT_TIMEOUT seconds of failsafe, and will always enter RTL after FS_LONG_TIMEOUT seconds of failsafe, regardless of the FS_LONG_ACTN setting. If FS_LONG_ACTN is set to 2 then instead of using RTL it will enter a FBWA glide with zero throttle.
+    // @Description: The action to take on a long (FS_LONG_TIMEOUT seconds) failsafe event. If the aircraft was in a stabilization or manual mode when failsafe started and a long failsafe occurs then it will change to RTL mode if FS_LONG_ACTN is 0 or 1, and will change to FBWA if FS_LONG_ACTN is set to 2. If the aircraft was in an auto mode (such as AUTO or GUIDED) when the failsafe started then it will continue in the auto mode if FS_LONG_ACTN is set to 0, will change to RTL mode if FS_LONG_ACTN is set to 1 and will change to FBWA mode if FS_LONG_ACTN is set to 2. 
     // @Values: 0:Continue,1:ReturnToLaunch,2:Glide
     // @User: Standard
     GSCALAR(long_fs_action,         "FS_LONG_ACTN",   LONG_FAILSAFE_ACTION),