Whats up with "RtlAbsoluteToSelfRelativeSD"?

(sorry about the double post, this is where I meant to post this in the
first place.

Hi all,

In some of my code I need to ensure that the Security Descriptors I am
dealing with are self relative. The function mentioned above seems to be
exactly what I need. Trouble is, it isn’t prototyped anywhere (except in
the GNU ntifs.h)

What’s the deal with this function? Is it just another undocumented API?
If so, is there an official way to do this?