Incorporate API review feedback for svn_fs.h and svn_ra_svn.h

* subversion/include/svn_fs.h
  (svn_fs_verify): rewrite the docstring for @a start and @a end;
                   add cross-references
  
* subversion/include/svn_ra_svn.h
  (svn_ra_svn_create_conn3): document @a zero_copy_limit parameter


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1462828 13f79535-47bb-0310-9956-ffa450edef68