api/types/client.go: documentation fix for ImageImportSource

Signed-off-by: Erik Hollensbe <github@hollensbe.org>

api/types/client.go

@@ -173,8 +173,8 @@ type ImageCreateOptions struct {
 
 // ImageImportSource holds source information for ImageImport
 type ImageImportSource struct {
-	Source     io.Reader // Source is the data to send to the server to create this image from (mutually exclusive with SourceName)
-	SourceName string    // SourceName is the name of the image to pull (mutually exclusive with Source)
+	Source     io.Reader // Source is the data to send to the server to create this image from. You must set SourceName to "-" to leverage this.
+	SourceName string    // SourceName is the name of the image to pull. Set to "-" to leverage the Source attribute.
 }
 
 // ImageImportOptions holds information to import images from the client host.