RtlIpv6AddressToStringExW - NtDoc

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

NTSYSAPI
NTSTATUS
NTAPI
RtlIpv6AddressToStringExW(
    _In_ PCIN6_ADDR Address,
    _In_ ULONG ScopeId,
    _In_ USHORT Port,
    _Out_writes_to_(*AddressStringLength, *AddressStringLength) PWSTR AddressString,
    _Inout_ PULONG AddressStringLength
    );

#endif

View code on GitHub

This function is documented in Windows SDK.