It reads:

"Creates a list of path of all selected rows. Additionally, if you are planning on modifying the model after calling this function, you may want to convert the returned list into a list of GtkTreeRowReferences. To do this, you can use gtk_tree_row_reference_new().

To free the return value, use:

g_list_foreach (list, gtk_tree_path_free, NULL);
g_list_free (list);"

that gtk_tree_path_free produces a warning while building if it's not casted to GFunc.
To fix the warning and the docs, just add (GFunc) in front:

g_list_foreach (list, (GFunc) gtk_tree_path_free, NULL);