message: ex-sliding: change diagram to suppress multi-line comment warnings.
diff: // restricted, but tangential movement is allowed, so the
  // trapezoid compresses into the notch.
  //
  -// /-----\
  -// normal constrained --->/ \ <--- boundary force (2)
  -// boundary (4) /---------\
  +// /-------+
  +// normal constrained --->/ | <--- boundary force (2)
  +// boundary (4) /---------+
  // ^
  // |
  // normal constrained boundary (1)
  