fuse: Add fuse-io-uring design documentation

[Add several documentation updates I had missed after
renaming functions and also fixes 'make htmldocs'.]

Signed-off-by: Bernd Schubert <bschubert@ddn.com>
Reviewed-by: Luis Henriques <luis@igalia.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>