message: Update descrition of ex15p to include the different error estimators.
diff: // At each outer iteration the right hand side function is changed
  // to mimic a time dependent problem. Within each inner iteration
  // the problem is solved on a sequence of meshes which are locally
  -// refined according to a simple ZZ error estimator. At the end
  -// of the inner iteration the error estimates are also used to
  +// refined according to a chosen error estimator. Currently there
  +// are three error estimators supported: A L2 formulation of the
  +// Zienkiewicz-Zhu error estimator (0), a Kelly error indicator (1)
  +// and a traditional Zienkiewicz-Zhu error estimator (2). At the
  +// end of the inner iteration the error estimates are also used to
  // identify any elements which may be over-refined and a single
  // derefinement step is performed. After each refinement or
  // derefinement step a rebalance operation is performed to keep
  