In latest doxygen docs generated on 2013-08-01, it's there on line 178:
explicit
unordered_map(const allocator_type& __a)
: _M_h(__a)
{ }
However, in the docs for 4.8.1 it's not there, which is the same as my local one. As as far as g++4.8 is concerned its not implemented.
Found the link to the patch. It was dated 2013-04-22, which was a little after the release of 4.8.