docs: it's not true that DISTCHECK_CONFIGURE_FLAGS is maintainer-reserved

docs: it's not true that DISTCHECK_CONFIGURE_FLAGS is maintainer-reserved

At least, not anymore since commit 'v1.11-372-g9760039' of 2001-06-10,
"distcheck: add support for AM_DISTCHECK_CONFIGURE_FLAGS".  See also
automake bug#8784.

* doc/automake.texi (Flag Variables Ordering): Do not report
'DISTCHECK_CONFIGURE_FLAGS' as a "variables that are only useful
to the maintainer that has no user counterpart": now it is a
user-reserved variable, its maintainer-reserved counterpart
being 'AM_DISTCHECK_CONFIGURE_FLAGS' .

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>