AP_ExternalAHRS: Clarify public interface design intent

* Discussions with Tridge on design intent for these accessors

Signed-off-by: Ryan Friedman <[email protected]>

libraries/AP_ExternalAHRS/AP_ExternalAHRS_backend.h

@@ -32,16 +32,22 @@ class AP_ExternalAHRS_backend {
     // Get model/type name
     virtual const char* get_name() const = 0;
 
-    // accessors for AP_AHRS
+    // Accessors for AP_AHRS
+
+    // If not healthy, none of the other API's can be trusted.
+    // Example: Serial cable is severed.
     virtual bool healthy(void) const = 0;
+    // The communication interface is up and the device has sent valid data.
     virtual bool initialised(void) const = 0;
     virtual bool pre_arm_check(char *failure_msg, uint8_t failure_msg_len) const = 0;
     virtual void get_filter_status(nav_filter_status &status) const {}
     virtual void send_status_report(class GCS_MAVLINK &link) const {}
 
-    // check for new data
+    // Check for new data.
+    // This is used when there's not a separate thread for EAHRS.
+    // This can also copy interim state protected by locking.
     virtual void update() = 0;
-    
+
 protected:
     AP_ExternalAHRS::state_t &state;
     uint16_t get_rate(void) const;