Doc: explain submodule.alternateErrorStrategy

Doc: explain submodule.alternateErrorStrategy

Commit 31224cbdc7 ("clone: recursive and reference option triggers
submodule alternates", 2016-08-17) taught Git to support the
configuration options "submodule.alternateLocation" and
"submodule.alternateErrorStrategy" on a superproject.

If "submodule.alternateLocation" is configured to "superproject" on a
superproject, whenever a submodule of that superproject is cloned, it
instead computes the analogous alternate path for that submodule from
$GIT_DIR/objects/info/alternates of the superproject, and references it.

The "submodule.alternateErrorStrategy" option determines what happens
if that alternate cannot be referenced. However, it is not clear that
the clone proceeds as if no alternate was specified when that option is
not set to "die" (as can be seen in the tests in 31224cbdc7). Therefore,
document it accordingly.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>