RtlAppendUnicodeStringToString - NtDoc

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

NTSYSAPI
NTSTATUS
NTAPI
RtlAppendUnicodeStringToString(
    _Inout_ PUNICODE_STRING Destination,
    _In_ PCUNICODE_STRING Source
    );

#endif

View code on GitHub

This function is documented in Windows Driver Kit.