Some do, some do not, it is not documented in the reference guide. Some can only be inferred from the example usage.

One function returning a C String in a RsslBuffer:

rsslDateTimeToString(&fidDateTimeBuf, RSSL_DT_DATETIME, &fidDateTimeValue);
printf("%s\n", fidDateTimeBuf.data);

Another returning only a buffer:

fidStateBuf.data = (char*)alloca(stateBufLen);
fidStateBuf.length = stateBufLen;
rsslStateToString(&fidStateBuf, &fidStateValue);
printf("%.*s\n", fidStateBuf.length, fidStateBuf.data);

Also some API completely ignore the apparent RsslBuffer standard signature and do return a C-String.

const char* rsslQosTimelinessToString(RsslUInt8 value)
{
    switch (value)
    {
    case RSSL_QOS_TIME_UNSPECIFIED:     return "Unspecified";
    case RSSL_QOS_TIME_REALTIME:        return "Realtime";
    case RSSL_QOS_TIME_DELAYED_UNKNOWN: return "DelayedByUnknown";
    case RSSL_QOS_TIME_DELAYED:         return "DelayedByTimeInfo";
    default:                            return "Unknown QosTimeliness";
    }
}

I have tried to debug the rsslStateToString function. The RsslBuffer.data always contains the null termination, even though the example code uses the RsslBuffer.length to print string.

I think rssl*ToString functions should generally add the 0 at the end of string of the rsslBuffer.data.