doc: discuss named references after locations.

doc: discuss named references after locations.

Reported by Hans Aberg at
<http://lists.gnu.org/archive/html/bug-bison/2011-05/msg00008.html>.
* NEWS (2.5.1): Document.
* doc/bison.texinfo (Named References): Because it discusses
locations in addition to semantic values, move this subsection out
of the section `Defining Language Semantics', where locations have
not yet been introduced, to be a new section after the following
section, `Tracking Locations'.