I just got the update too. No description of what it did (of course!) I checked the regular S5 forum and, interestingly, no one has motioned anything about getting an update recently on the regular S5.
I wish I took a screen shot of the About page so that I could at least see the versions of the various things I had before the update and compare to what I have now!
Hopefully, someone will figure out what the heck this update did!
I did get the update, my Baseband version is: G870AUCU1ANG3. It's odd that AT&T won't give it to you, although it's not necessary, at least you can still use Towelroot, I haven't rooted this one, so no loss.