* improve the doc of data feeder related APIs (#20515)

* improve data feeder related API

* fix doc of default_main_program, multiprocess_reader (#20536)

* fix doc of default_main_program, multiprocess_reader

* update API.spec

* fix comment

* update data feeder API sample, change fluid.layers.data to fluid.data (#20568)

* update data feeder API sample, fluid.layers.data => fluid.data

* update API.spec

* update API.spec