docs: korn shells can have $? > 256 for signal-terminated children

docs: korn shells can have $? > 256 for signal-terminated children

Some Korn shells, when a child process dies due to signal number
n, can leave in $? an exit status of 256+n, instead of the more
common 128+n.  See also Austin Group issue 0000051:
  <http://www.austingroupbugs.net/view.php?id=51>

* doc/autoconf.texi (Signal handling): Document the described Korn
Shell behaviour, and some of its possible shortcomings.

Suggestion by Eric Blake.