Fix up "@since" markers in doc strings.

* subversion/include/svn_fs.h
  (SVN_FS_TXN_CLIENT_DATE, svn_fs_lock_callback_t):
    Mark as new in 1.9.

* subversion/include/svn_path.h
  (svn_path_illegal_path_escape): Mark as new in 1.8, rather than 1.9, as
    it was first released in 1.8.0 (via r1482136).

* subversion/include/svn_types.h
  (svn_null_pointer_constant_stdarg_sentinel_t, SVN_VA_NULL):
    Mark as new in 1.9.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1604735 13f79535-47bb-0310-9956-ffa450edef68