also document handlers sharing a same source. TW#13454, https://github.com/nodemcu/nodemcu-firmware/issues/1874 Breaking change: handles assigned to a same source should have the same flag now.
Merges https://github.com/espressif/esp-idf/pull/701
This is to resolve issue reported in https://github.com/espressif/esp-idf/issues/130.