parse.lac: document.

parse.lac: document.

* NEWS (2.5): Add entry for LAC, and mention LAC in entry for
other corrections to verbose syntax error messages.
* doc/bison.texinfo (Decl Summary): Rewrite entries for
lr.default-reductions and lr.type to be clearer, to mention
%nonassoc's effect on canonical LR, and to mention LAC.  Add entry
for parse.lac.
(Glossary): Add entry for LAC.
(cherry picked from commit fcf834f9ecf080784b741782f4206df1e1a2957a)

Conflicts:

	doc/bison.texinfo