accounting-guide: add "queue", "age" factors

Problem: The accounting guide does not list "queue" or "age" as factors
considered by the plugin when calculating a job's priority.

Add both of these factors to the listed description of factors used
when calculating a job's priority in the priority plugin.

doc/guide/accounting-guide.rst

@@ -316,6 +316,12 @@ the multi-factor priority plugin are:
   consumed. See the :ref:`Glossary definition <glossary-section>` for a more
   detailed explanation of how fair-share is utilized within flux-accounting.
 
+* **queue**: an integer weight associated with submitting a job to a certain
+  queue.
+
+* **age**: how long a job has been able to be scheduled but could not due to an
+  external constraint (e.g a resource constraint).
+
 * **urgency**: a user-controlled factor to prioritize their own jobs.
 
 In addition to generating an integer priority for submitted jobs in a Flux