* "Content-Type: application/json" header if @a add_json is set.
*
* @param ctx context to execute the job in
- * @param eh curl easy handle for the request, will
- * be executed AND cleaned up
+ * @param eh curl easy handle for the request, will be executed AND
+ * cleaned up. NOTE: the handle should _never_ have gotten
+ * any headers list, as that would then be ovverridden by
+ * @a jcc. Therefore, always pass custom headers as the
+ * @a job_headers parameter.
* @param job_headers extra headers to add for this request
* @param jcc callback to invoke upon completion
* @param jcc_cls closure for @a jcc