RtlInitString - NtDoc

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

FORCEINLINE VOID RtlInitString(
    _Out_ PSTRING DestinationString,
    _In_opt_ PCSTR SourceString
    )
{
    if (SourceString)
        DestinationString->MaximumLength = (DestinationString->Length = (USHORT)strlen(SourceString)) + sizeof(ANSI_NULL);
    else
        DestinationString->MaximumLength = DestinationString->Length = 0;

    DestinationString->Buffer = (PCHAR)SourceString;
}

#endif
#endif

View code on GitHub
#ifndef _NTRTL_H
#ifndef PHNT_NO_INLINE_INIT_STRING
// ...
#else

NTSYSAPI
VOID
NTAPI
RtlInitString(
    _Out_ PSTRING DestinationString,
    _In_opt_ PCSTR SourceString
    );

#endif
#endif

View code on GitHub

This function is documented in Windows Driver Kit.