doc/fsck: clarify --connectivity-only behavior

doc/fsck: clarify --connectivity-only behavior

On reading this again, there are two things that were not immediately
clear to me:

  - we do still check links to blobs, even though we don't open the
    blobs themselves

  - we do not do the normal fsck checks, even for non-blob objects we do
    open

Let's reword it to make these points a little more clear.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>