The man page for
7z simply says
Do not use "-r" because this flag does not do what you think.
The only documentation I can find basically restates the same thing.
-r[-|0] Recurse subdirectories (CAUTION: this flag does not do what you think, avoid using it)
So, what does it do, and why does the author of this documentation think I think it does something else?
This is 7zip 9.04 on Debian Squeeze (i.e. pretty old).