docs: serial-tests are not deprecated, just discouraged

docs: serial-tests are not deprecated, just discouraged

We don't plan to remove support for them, nor to have the serial-tests
option give any kind of runtime warning, so don't alarm the users
still using serial tests with pointless "deprecation" or "obsolescence"
warnings.

Fixes automake bug#13478.

See also:
<http://lists.gnu.org/archive/html/automake/2013-01/msg00058.html>

* doc/automake.texi: Adjust.
* THANKS: Update.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>