connected: document connectivity in partial clones

connected: document connectivity in partial clones

In acb0c57260 ("fetch: support filters", 2017-12-08), check_connected()
was extended to allow objects to either be promised to be available (if
the repository is a partial clone) or to be present; previously, this
function required the latter. However, this change was not reflected in
the documentation of that function. Update the documentation
accordingly.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>