* doc/autoconf.texi (Particular Programs): Do not promise that
we always prefer the GNU version of the program, and that we
search according to PATH; both rules can have exceptions.
Update description of AC_PROG_GREP, AC_PROG_EGREP, AC_PROG_FGREP,
AC_PROG_SED. Move descriptions of limitations
to the Limitations of Usual Tools section.
(Limitations of Usual Tools) <sed>: Mention script length
limitations with Solaris /usr/ucb/sed.
<grep>: Fix wording for empty alternative. Mention that -c and
-l should not be combined, and that -E and -F should not be
combined.