RtlUshortByteSwap - NtDoc

Native API online documentation, based on the System Informer (formerly Process Hacker) phnt headers
#ifndef _NTRTL_H
// Byte swap routines.
#ifndef PHNT_RTL_BYTESWAP

#define RtlUshortByteSwap(_x) _byteswap_ushort((USHORT)(_x))

#endif
#endif

View code on GitHub
#ifndef _NTRTL_H
// Byte swap routines.
#ifndef PHNT_RTL_BYTESWAP
// ...
#else

NTSYSAPI
USHORT
FASTCALL
RtlUshortByteSwap(
    _In_ USHORT Source
    );

#endif
#endif

View code on GitHub

This function is documented in Windows Driver Kit.