Hello,
Chapter 6, section "svnserve Configuration Reference" - "General configuration" :
http://svnbook.red-bean.com/en/1.8/svn.serverconfig.svnserve.html#svn.serverconfig.svnserve.ref.
I don't understand the 2nd paragraph of "hooks-env" directive. It seems to me like it is a wrong copy-paste from the previous directive "groups-db".
Oops! A bad copy-and-paste, indeed. Removed in r5576. Thanks!