Add comments to help communicate the defects in the simple mesh (with…

… ONE face) contained in the file bad_geometry_volume_negative.obj

geometry/test/bad_geometry_volume_negative.obj

@@ -1,12 +1,18 @@
 # File: bad_geometry_volume_negative.obj
-# The geometry in this file produces invalid mass and inertia properties
-# due to improper winding (order of the vertices) in its one face.
-# Drake calculates a negative volume for this mesh (volume = -1.0).
+# The geometry in this file does not describe a closed triangular mesh (it only
+# has ONE face) and Drake's mass properties calculator produces an invalid
+# volume, invalid mass, invalid center of mass, and invalid inertia matrix.
+# An additional defect (which makes invalidity easier to detect) is that its one
+# face has a winding (order of its vertices) that results in Drake calculating
+# a negative volume for this mesh (volume = -1.0).
 # Related files: bad_geometry_volume_zero.obj and bad_geometry_corrected.obj
 # Related: issue #21924 [github.com/RobotLocomotion/drake/issues/21924].
 # ------------------------------------------------------------------------------
 
 # The 3 vertices below together with the origin (0, 0, 0) form a tetrahedron.
+# Drake happens to use this single tetrahedron to calculate volume and other
+# mass proeprties. However, this mesh only has ONE face, so its bounding box
+# indicates a volume = 0, which is another way to detect mesh defects.
 v 1.0  0.0  0.0
 v 1.0  2.0  0.0
 v 1.0  0.0  3.0