docs: signal-related bugs and incompatibilities for the shells

docs: signal-related bugs and incompatibilities for the shells

* doc/autoconf.texi (Signal handling): New paragraph.
(@menu at "Portable Shell", @detailmenu): Update.

Motivated by recent discussion on the bug-autoconf list, as well
as work in the automake testsuite:
 <https://lists.gnu.org/archive/html/bug-autoconf/2011-09/msg00003.html>
 <https://lists.gnu.org/archive/html/bug-autoconf/2011-09/msg00004.html>
 <http://lists.gnu.org/archive/html/automake-patches/2011-09/msg00066.html>