On the log-addressing branch: Follow-up to r1511365.
Doc string only change.

* subversion/libsvn_fs_fs/index.c
  (): add some commentary

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/log-addressing@1511368 13f79535-47bb-0310-9956-ffa450edef68