On the fsfs-format7 branch:  follow-up to r1438412.

* subversion/libsvn_fs_fs/low_level.h
  (svn_fs_fs__read_changes): update docstring

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/fsfs-format7@1438449 13f79535-47bb-0310-9956-ffa450edef68