Cleanup: document id_lib_extern()

Add documentation for `id_lib_extern()`.

No functional changes.

Pull Request: https://projects.blender.org/blender/blender/pulls/148006

source/blender/blenkernel/BKE_lib_id.hh

@@ -615,6 +615,17 @@ void BKE_libblock_management_main_remove(Main *bmain, void *idv);
 void BKE_libblock_management_usercounts_set(Main *bmain, void *idv);
 void BKE_libblock_management_usercounts_clear(Main *bmain, void *idv);
 
+/**
+ * Flag this linked ID as directly used by another ID in the current blend file.
+ *
+ * If the ID was marked as indirectly/weakly linked, those flags are cleared.
+ *
+ * This is a no-op when `id` is `nullptr` or not linked.
+ *
+ * This status is rechecked for the whole Main data-base as a step of pre-blendfile writing (see
+ * #write_id_direct_linked_data_process_cb() and its usage in #write_file_handle). This ensures
+ * that no reference to indirectly used IDs are kept in the written blendfile.
+ */
 void id_lib_extern(ID *id);
 void id_lib_indirect_weak_link(ID *id);
 /**