RtlInitStringEx - NtDoc

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

NTSYSAPI
NTSTATUS
NTAPI
RtlInitStringEx(
    _Out_ PSTRING DestinationString,
    _In_opt_z_ PCSZ SourceString
    );

#endif
#endif

View code on GitHub

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