message: Copy the doc from FilteredComplex::simplex to the Simplex_tree.
diff: @@ -391,13 +391,13 @@ class Simplex_tree {
  return sh->second.key();
  }
  - /** \brief Returns the simplex associated to a key.
  + /** \brief Returns the simplex that has index idx in the filtration.
  *
  * The filtration must be initialized.
  * \pre SimplexTreeOptions::store_key
  */
  - Simplex_handle simplex(Simplex_key key) const {
  - return filtration_vect_[key];
  + Simplex_handle simplex(Simplex_key idx) const {
  + return filtration_vect_[idx];
  }
  /** \brief Returns the filtration value of a simplex.
  