Clarify use of 206 status code for node health.

The 206 return code for the node health endpoint does not take into
account the situation where the consensus node may be synced by the
execution node is optimistic.  Clarify that this situation should return
the 206 status code.

apis/node/health.yaml

@@ -17,7 +17,7 @@ get:
     "200":
       description: Node is ready
     "206":
-      description: Node is syncing but can serve incomplete data
+      description: Either the consensus node is syncing, or its execution node is optimistic, so data served may be incorrect
     "400":
       description: Invalid syncing status code
     "503":