NtFlushBuffersFileEx - NtDoc

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

NTSYSCALLAPI
NTSTATUS
NTAPI
NtFlushBuffersFileEx(
    _In_ HANDLE FileHandle,
    _In_ ULONG Flags,
    _In_reads_bytes_(ParametersSize) PVOID Parameters,
    _In_ ULONG ParametersSize,
    _Out_ PIO_STATUS_BLOCK IoStatusBlock
    );

#endif
#endif

View code on GitHub
#ifndef _NTZWAPI_H

NTSYSCALLAPI
NTSTATUS
NTAPI
ZwFlushBuffersFileEx(
    _In_ HANDLE FileHandle,
    _In_ ULONG Flags,
    _In_reads_bytes_(ParametersSize) PVOID Parameters,
    _In_ ULONG ParametersSize,
    _Out_ PIO_STATUS_BLOCK IoStatusBlock
    );

#endif

View code on GitHub

This function is documented in Windows Driver Kit here and here.