Currently the docs say, "The result is calculated in a way which is accurate for x near zero."

That is somewhat vague.  Some quick tests show that it is often more accurate than log() for the whole range of 0.0 < x < 1.0; however, for x < 0.0 it is always worse.

Since the function is just a pass-through to the C library, we probably can't make any guarantees.  On the other hand, it is hard to know when this function is preferred without some guidance.