doc: document fundamental issue with ERROR/tunnel

aggregatetunnelmetrics/spec/fieldtesting.py

@@ -66,6 +66,16 @@ class Entry:
 
     def is_tunnel_error_measurement(self) -> bool:
         """Return whether this is a failed tunnel measurement"""
+        # TODO(bassosimone): this method is used wrongly and my assumption
+        # about the semantics of this error is also completely wrong.
+        #
+        # The `ERROR/tunnel` error means that anything went wrong during the
+        # lifecycle of a tunnel AND DOES NOT MEAN that we failed to start
+        # the tunnel itself.
+        #
+        # As a result, we need to revisit the logic.
+        #
+        # See https://0xacab.org/solitech/monitoring/-/merge_requests/6#note_1247230.
         return self.tunnel == "ERROR/tunnel"