cmd/upload: document "raw" default for --format argument

When first using the tool, based on the `--help` output I did not
realize that `raw` was a supported format. Then, upon stumbling on a
GitHub issue that documents this format as being able to stream larger
images directly to disk, I found out that specifying `--format raw`
does not work, and leads to a failure relatively late in the image
upload process.

This documents that, when not specifying `--format`, a default format
of `raw` is assumed.

cmd/upload.go

@@ -127,7 +127,7 @@ func init() {
 		cobra.FixedCompletions([]string{string(hcloudimages.CompressionBZ2), string(hcloudimages.CompressionXZ)}, cobra.ShellCompDirectiveNoFileComp),
 	)
 
-	uploadCmd.Flags().String(uploadFlagFormat, "", "Format of the image. [choices: qcow2]")
+	uploadCmd.Flags().String(uploadFlagFormat, "", "Format of the image. [default: raw, choices: qcow2]")
 	_ = uploadCmd.RegisterFlagCompletionFunc(
 		uploadFlagFormat,
 		cobra.FixedCompletions([]string{string(hcloudimages.FormatQCOW2)}, cobra.ShellCompDirectiveNoFileComp),

docs/reference/cli/hcloud-upload-image_upload.md

@@ -36,7 +36,7 @@ hcloud-upload-image upload (--image-path=<local-path> | --image-url=<url>) --arc
       --architecture string     CPU architecture of the disk image [choices: x86, arm]
       --compression string      Type of compression that was used on the disk image [choices: bz2, xz]
       --description string      Description for the resulting image
-      --format string           Format of the image. [choices: qcow2]
+      --format string           Format of the image. [default: raw, choices: qcow2]
   -h, --help                    help for upload
       --image-path string       Local path to the disk image that should be uploaded
       --image-url string        Remote URL of the disk image that should be uploaded