docs: mention another issue with variable expansion

docs: mention another issue with variable expansion

In particular, see http://austingroupbugs.net/view.php?id=221
and http://austingroupbugs.net/view.php?id=255.

* doc/autoconf.texi (Shell Substitutions) <${var+value}>: New
subsection.
<${var=literal}>: Tweak wording.  Add mention of an ambiguity
allowed by POSIX.
* tests/torture.at (Substitute and define special characters):
Make test more robust; here, the outer "" is in a here-doc, and
does not violate the quoting rules of thumb just documented.

Signed-off-by: Eric Blake <eblake@redhat.com>