Talk about URL support and the real meaning of `-` in the latest `fromSrc` API documentation.

Signed-off-by: Jean-Paul Calderone <exarkun@twistedmatrix.com>

docs/sources/reference/api/docker_remote_api_v1.16.md

@@ -861,7 +861,8 @@ Create an image, either by pulling it from the registry or by importing it
 Query Parameters:
 
 -   **fromImage** – name of the image to pull
--   **fromSrc** – source to import, - means stdin
+-   **fromSrc** – source to import.  The value may be a URL from which the image
+        can be retrieved or `-` to read the image from the request body.
 -   **repo** – repository
 -   **tag** – tag
 -   **registry** – the registry to pull from