AC_Fence: add comment re check_fence method

libraries/AC_Fence/AC_Fence.cpp

@@ -111,6 +111,7 @@ bool AC_Fence::pre_arm_check() const
 }
 
 /// check_fence - returns the fence type that has been breached (if any)
+///     curr_alt is the altitude above home in meters
 uint8_t AC_Fence::check_fence(float curr_alt)
 {
     uint8_t ret = AC_FENCE_TYPE_NONE;

libraries/AC_Fence/AC_Fence.h

@@ -52,6 +52,7 @@ public:
     ///
 
     /// check_fence - returns the fence type that has been breached (if any)
+    ///     curr_alt is the altitude above home in meters
     uint8_t check_fence(float curr_alt);
 
     /// get_breaches - returns bit mask of the fence types that have been breached