docs: correct information about reftable

docs: correct information about reftable

Our description of the reftable format is that it is experimental and
subject to change, but that is no longer true.  Remove this statement so
as not to mislead users.

In addition, the documentation says that the files format is the
default, but that is not true if breaking changes mode is on.  Correct
this information with a conditional.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>