docs: deprecate .log -> .html conversion by parallel-tests

docs: deprecate .log -> .html conversion by parallel-tests

That feature is to be deprecated in the 1.11.x series, and
removed in the 1.12 release.

Reference:
  <http://lists.gnu.org/archive/html/automake/2012-01/msg00005.html>

* doc/automake.texi (Simple Tests using parallel-tests): Deprecate
the '.log' -> '.html' conversion and the targets 'check-html' and
'recheck-html'.  Related rewording and reformatting.
* NEWS (Future backward-incompatibilities): Update.