doc: bt_field_class...get_mapping_labels...(): clarify RV's validity

doc: bt_field_class...get_mapping_labels...(): clarify RV's validity

The label array which
bt_field_class_enumeration_unsigned_get_mapping_labels_for_value() and
bt_field_class_enumeration_signed_get_mapping_labels_for_value() return
remains valid as long as:

* The enumeration field class is not modified.

* You don't call the same function again with the same enumeration
  field class.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I7b197ab5e176c77f4418d23b12e194c6477e5cf8
Reviewed-on: https://review.lttng.org/c/babeltrace/+/3226