Improve cache variable documentation.

Improve cache variable documentation.

* doc/autoconf.texi (AC_PROG_AWK, AC_PROG_GREP, AC_PROG_EGREP,
AC_PROG_FGREP, AC_PROG_INSTALL, AC_PROG_MKDIR_P, AC_PROG_LEX,
AC_PROG_YACC, AC_CHECK_PROG, AC_CHECK_PROGS, AC_PATH_PROG,
AC_PATH_PROGS): Don't suggest to use the cache variable, only to
override it, or preferably, a non-cache variable associated with
the test.
(AC_PROG_SED): Likewise. Fix name of cache variable.
(AC_FUNC_GETMNTENT): Fix name cache variable.
(AC_FUNC_LSTAT): Fix typo.

Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>