Indicate which forest_geometry functions are only linear approximations and which actually use the registered geometry.

[lukasdreyer]

For example, the function t8_forest_element_volume is such a linear approximation, but this is not recognizable from the function name.

/** Compute the volume of an element if a geometry for this tree is registered in the forest's cmesh.
 * This is only an approximation.
 * \param [in]      forest     The forest.
 * \param [in]      ltree_id   The forest local id of the tree in which the element is.
 * \param [in]      element    The element.
 * \return                     The diameter of the element.
 * \note                       This function assumes d-linear interpolation for the
 *                             tree vertex coordinates.
 *                             \a forest must be committed when calling this function.
 */
double
t8_forest_element_volume (t8_forest_t forest, t8_locidx_t ltreeid, const t8_element_t *element);