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>

Documentation/git-fsck.txt

@@ -62,9 +62,13 @@ index file, all SHA-1 references in `refs` namespace, and all reflogs
 	with --no-full.
 
 --connectivity-only::
-	Check only the connectivity of tags, commits and tree objects. By
-	avoiding to unpack blobs, this speeds up the operation, at the
-	expense of missing corrupt objects or other problematic issues.
+	Check only the connectivity of reachable objects, making sure
+	that any objects referenced by a reachable tag, commit, or tree
+	is present. This speeds up the operation by avoiding reading
+	blobs entirely (though it does still check that referenced blobs
+	exist). This will detect corruption in commits and trees, but
+	not do any semantic checks (e.g., for format errors). Corruption
+	in blob objects will not be detected at all.
 
 --strict::
 	Enable more strict checking, namely to catch a file mode