NtCancelWaitCompletionPacket - NtDoc

Native API online documentation, based on the System Informer (formerly Process Hacker) phnt headers
#ifndef _NTIOAPI_H
// Wait completion packet
#if (PHNT_VERSION >= PHNT_WIN8)

NTSYSCALLAPI
NTSTATUS
NTAPI
NtCancelWaitCompletionPacket(
    _In_ HANDLE WaitCompletionPacketHandle,
    _In_ BOOLEAN RemoveSignaledPacket
    );

#endif
#endif

View code on GitHub
#ifndef _NTZWAPI_H

NTSYSCALLAPI
NTSTATUS
NTAPI
ZwCancelWaitCompletionPacket(
    _In_ HANDLE WaitCompletionPacketHandle,
    _In_ BOOLEAN RemoveSignaledPacket
    );

#endif

View code on GitHub

This function is documented in Windows SDK.