Explain a subtlety of client handling for "download" batch action

The LFS client uses the presence of the "download" action on an "upload" request to deduce that the object content is already present on the server: the object content must be present, or the server could not offer a "download" action. Since the server already knows the object content, the client does not need to upload a new copy and can skip the transfer step.

Make this interaction between the client and server explicit in the documentation.

Author: Evan Priestley

docs/api/http-v1-batch.md

@@ -105,7 +105,10 @@ when the object has not been previously uploaded.
 successfully uploading an object.  This is an optional relation for the case that
 the server has not verified the object.
 * `download` - This relation describes how to download the object content.  This
-only appears if an object has been previously uploaded.
+only appears if an object has been previously uploaded.  If object content is
+already available, the server should provide this action even if the request
+operation is `upload`, because the client can use the presence of this action to
+avoid duplicate uploads of object content which is already known to the server.
 
 An action can optionally include an `expires_at`, which is an ISO 8601 formatted
 timestamp for when the given action expires (usually due to a temporary token).