[struct_pack][doc] fix cn api document #864

Job	Run time
ubuntu_clang_for_liburing (Release, OFF, ON)	3m 1s
ubuntu_clang_pb (Release, OFF, ON)	2m 41s
ubuntu_clang_for_liburing (Debug, OFF, ON)	3m 9s
ubuntu_clang_pb (Release, OFF, OFF)	2m 59s
ubuntu_clang (Release, OFF, ON)	5m 55s
ubuntu_clang (Release, OFF, OFF)	5m 9s
ubuntu_clang_pb (Debug, OFF, ON)	2m 27s
ubuntu_clang_pb (Debug, OFF, OFF)	2m 23s
ubuntu_clang10 (Release)	2m 13s
ubuntu_clang (Debug, OFF, ON)	4m 41s
ubuntu_clang (Debug, OFF, OFF)	4m 26s
ubuntu_clang10 (Debug)	1m 43s
	40m 47s

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