message: Fix doc of Simplex_tree about keys
diff: @@ -463,7 +463,7 @@ class Simplex_tree {
  public:
  /** \brief Returns the key associated to a simplex.
  *
  - * The filtration must be initialized.
  + * If no key has been assigned, returns `null_key()`.
  * \pre SimplexTreeOptions::store_key
  */
  static Simplex_key key(Simplex_handle sh) {
  @@ -473,7 +473,6 @@ class Simplex_tree {
  /** \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 idx) const {
  return filtration_vect_[idx];
  @@ -509,8 +508,7 @@ class Simplex_tree {
  return Dictionary_it(nullptr);
  }
  - /** \brief Returns a key different for all keys associated to the
  - * simplices of the simplicial complex. */
  + /** \brief Returns a fixed number not in the interval [0, `num_simplices()`). */
  static Simplex_key null_key() {
  return -1;
  }
  @@ -856,11 +854,9 @@ class Simplex_tree {
  public:
  /** \brief Initializes the filtrations, i.e. sort the
  - * simplices according to their order in the filtration and initializes all Simplex_keys.
  + * simplices according to their order in the filtration.
  *
  - * After calling this method, filtration_simplex_range() becomes valid, and each simplex is
  - * assigned a Simplex_key corresponding to its order in the filtration (from 0 to m-1 for a
  - * simplicial complex with m simplices).
  + * After calling this method, filtration_simplex_range() becomes valid.
  *
  * Will be automatically called when calling filtration_simplex_range()
  * if the filtration has never been initialized yet. */
  