Fix some wording problems noted by Paolo Bonzini in:
http://lists.gnu.org/archive/html/autoconf-patches/2007-01/msg00077.html
* doc/autoconf.texi (Signed Overflow Examples): Give more
discussion about the allow_superuser_privileges example,
and change it a bit to make things clearer.
(Optimization and Wraparound): Clarify whether the compiler
will generate an infinite loop for the example derived from
Autoconf's mktime test.
(Signed Overflow Advice): Say that -ftrapv is meant for debugging.
Also, clarify unsigned multiplication overflow.