manual: document bare-user-only repo mode

Closes: #903
Approved by: cgwalters

docs/manual/repo.md

@@ -81,6 +81,13 @@ The `bare-user` mode is useful for build systems that run as non-root
 but want to generate root-owned content, as well as non-root container
 systems.
 
+There is a variant to the `bare-user` mode called `bare-user-only`. Unlike
+`bare-user`, neither ownership nor extended attributes are stored. These repos
+are meant to to be checked out in user mode (with the `-U` flag), where this
+information is not applied anyway. The main advantage of `bare-user-only` is
+that repos can be stored on filesystems which do not support extended
+attributes, such as tmpfs.
+
 In contrast, the `archive-z2` mode is designed for serving via plain
 HTTP.  Like tar files, it can be read/written by non-root users.