btrfs-progs: document json format str

The 'str' type was added in ecbb6a7fcd ("btrfs-progs: add json
formatter for escaped string") but not documented. It should be used
e.g. for paths or strings from unknown origin.

Signed-off-by: David Sterba <dsterba@suse.com>

common/format-output.h

@@ -29,6 +29,8 @@ struct rowspec {
 	 *   (values: va_args)
 	 * - bool: print boolean value true/false (unquoted, native type)
 	 *   (values: bool or int)
+	 * - str: string type with quoted special characters (use for unknown input source)
+	 *   (values: const char *)
 	 * - uuid: format UUID as text
 	 *   (value: u8 *uuid)
 	 * - time-long: pretty print timestamp, including timezone