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>

connected.h

@@ -51,9 +51,9 @@ struct check_connected_options {
 #define CHECK_CONNECTED_INIT { 0 }
 
 /*
- * Make sure that our object store has all the commits necessary to
- * connect the ancestry chain to some of our existing refs, and all
- * the trees and blobs that these commits use.
+ * Make sure that all given objects and all objects reachable from them
+ * either exist in our object store or (if the repository is a partial
+ * clone) are promised to be available.
  *
  * Return 0 if Ok, non zero otherwise (i.e. some missing objects)
  *