gitremote-helpers(1): clarify refspec behaviour

The documentation says that "If no 'refspec' capability is advertised,
there is an implied `refspec *:*`" but this is only the case for the
"import" command.

Since there is a comment in transport-helper.c indicating that this
default is for historical reasons, change the documentation to clarify
that a refspec should always be specified.

Signed-off-by: John Keeping <john@keeping.me.uk>
Acked-by: Sverre Rabbelier <srabbelier@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/gitremote-helpers.txt

@@ -174,8 +174,8 @@ ref.
 This capability can be advertised multiple times.  The first
 applicable refspec takes precedence.  The left-hand of refspecs
 advertised with this capability must cover all refs reported by
-the list command.  If no 'refspec' capability is advertised,
-there is an implied `refspec *:*`.
+the list command.  If a helper does not need a specific 'refspec'
+capability then it should advertise `refspec *:*`.
 
 'bidi-import'::
 	This modifies the 'import' capability.