comment update

apriltag_ros/config/tags.yaml

@@ -3,6 +3,8 @@
 # ## General remarks
 #
 # - All length in meters
+# - 'size' refers to the length of the shared border between solid black and solid white rectangle.
+#   See https://github.com/AprilRobotics/apriltag/wiki/AprilTag-User-Guide#pose-estimation for details.
 # - Ellipsis (...) signifies that the previous element can be repeated multiple times.
 #
 # ## Standalone tag definitions