doc: clean up `Named References' a little.

doc: clean up `Named References' a little.

* doc/bison.texinfo (Named References): Rename section to match
the node name, and update all cross references.  Don't use the
acronyms RHS and LHS given that we don't expand them anywhere in
the manual.  Try to use @samp and @code properly.  Mention that
named references are experimental as we've already stated in NEWS.
(Destructor Decl): Expand the only remaining use of RHS or LHS.