On the 1.8.x branch, doxygen-ify two docstrings.

This was done on trunk as part of r1483532, and as a documentation fix does not
require a vote.

* subversion/include/svn_hash.h
  (svn_hash_gets, svn_hash_sets): Mark docstrings for doxygen.


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/1.8.x@1483971 13f79535-47bb-0310-9956-ffa450edef68