[struct_pack][doc] fix cnapi document (#563) #865

Job	Run time
ubuntu_clang (Release, OFF, ON)	6m 4s
ubuntu_clang (Release, OFF, OFF)	6m 3s
ubuntu_clang (Debug, OFF, ON)	5m 26s
ubuntu_clang (Debug, OFF, OFF)	5m 6s
ubuntu_clang_for_liburing (Release, OFF, ON)	3m 54s
ubuntu_clang_for_liburing (Debug, OFF, ON)	3m 11s
ubuntu_clang_pb (Release, OFF, ON)	3m 59s
ubuntu_clang_pb (Release, OFF, OFF)	4m 2s
ubuntu_clang_pb (Debug, OFF, ON)	3m 4s
ubuntu_clang_pb (Debug, OFF, OFF)	3m 15s
ubuntu_clang10 (Release)	2m 34s
ubuntu_clang10 (Debug)	1m 57s
	48m 35s

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