Thanks, but... I'm at a loss. Been trying to make head or tails out of this "Documents & Data" for a couple of weeks now, since I found it chewing up 29gb out of my 64gb phone. I'm pretty sure I know the solution -- factory reset, reload apps, etc., but am still hopeful there's a less painful way to correct this. Searching the Apple forums and Google, it seems no one has really found a good answer, however.
After plugging my 64gb iPhone 6s in and bringing iTunes up about a couple of weeks ago, I noticed with a bit of alarm that this "Documents & Data" category was chewing up a little over 29gb (!!!) of space. So, I went through, deleted a few apps, cleared out some old messages and some old voicemails, etc..
Unplugged the phone, restarted, plugged the phone back in. Now, iTunes was reporting that Documents & Data were taking up 19gb of space. Quite an improvement, but that's still a pretty good chunk... So, figured I'd back up and delete my old photos (about 1gb worth) off the phone. Tracked down and found another small app I could delete. Restarted and plugged the phone back in. iTunes fires back up, and now reports Documents & Data are taking up 26gb (!!!???). This made no sense to me whatsoever as I didn't open any apps or install anything, but that's where it still stands today -- still around 26gb.
At this point, I went in to Settings -> Storage & iCloud Usage -> Manage Storage (under Storage), went in to each app listed, and tallied everything up. This is what I came to:
Apps: 9.91gb
Data & Documents: 9.32gb
This included native iOS apps like Photos & Camera, Music, Messages, etc. I know that's not 100% accurate, but I would think it should at least be in the ballpark. iTunes, for comparison, reports this:
Apps: 7.82gb
Data & Documents: 26.22gb
I don't know. I'll give it until this weekend to see if I can figure anything out... if I don't by then, I'll just get the wipe and reload over and done with. If I do have an epiphany, I'll be sure to post here what I figure out.