The only documentation about googles \'repo\' tool for android repository management was:
https://sites.google.com/a/android.com/opensource/download/using-repo
Documentation about repo is a bit hard to come about, as you have discovered.
In addition to your link, the repo help <subject> commands are very useful. E.g. repo help manifest and repo help sync are quite well described.
Also if you look in your .repo/repo/docs/manifest-format.txt there are very decent description of the manifest system used by the Android repo program.
If you discover additional resources of information on repo I would be very interested in hearing about it. :)