Answer: it's a eFFin sandbox for Flatpaks and, possibly, Snaps:
Yes, it is a directory.
It contains a FUSE-backed filesystem for the Flatpak Documents portal. Files that have been exported into it from the host system are made available to Flatpak applications. This occurs when a Flatpak application uses the FileChooser portal, for instance.
The filesystem should not take up any significant amount of memory, and it isn't present on disk at all. Any files you might find in it (if there are any at all) are essentially proxies for the corresponding files on the outside filesystem. That is, the FUSE filesystem simply passes file operations on it through to the files that were actually exported; it doesn't store any file content itself.
It's an answer from Reddit question "What in the world is /run/user/1000/doc ?"
https://www.reddit.com/r/linuxquestions/comments/18c06ae/what_in_the_world_is_runuser1000doc/
In conclusion, Canonical decided to hijack my filesystem with their ¢rap, because God forbid that user can use Home or tmp folder.
Truly first thing after installing any Linux is to completely remove Flatpak and Snap.