Adds settings for worker retry time, with what should be sensible default

docs/configuration.rst

@@ -540,6 +540,10 @@ PAPERLESS_WORKER_TIMEOUT=<num>
     large documents within the default 1800 seconds. So extending this timeout
     may prove to be useful on weak hardware setups.
 
+PAPERLESS_WORKER_RETRY=<num>
+    If PAPERLESS_WORKER_TIMEOUT has been configured, the retry time for a task can
+    also be configured.  By default, this value will be set to 10s more than the
+    worker timeout.  This value should never be set less than the worker timeout.
 
 PAPERLESS_TIME_ZONE=<timezone>
     Set the time zone here.