RtlSetBit - NtDoc

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

NTSYSAPI
VOID
NTAPI
RtlSetBit(
    _In_ PRTL_BITMAP BitMapHeader,
    _In_range_(<, BitMapHeader->SizeOfBitMap) ULONG BitNumber
    );

#endif
#endif

View code on GitHub

This function is documented in Windows Driver Kit.