doc: add note about --scrub, VACUUM SQLite command

Problem: The flux-accounting guide does not make a note of the --scrub
optional argument or running VACUUM on the database file if space needs
to be reclaimed.

Add a note to the flux-accounting guide about what the --scrub optional
argument does and how to tune it. Make a note about connecting to the
database file and running VACUUM if we want to clean up and optimize
the space occupied by the file.

doc/guide/accounting-guide.rst

@@ -171,6 +171,32 @@ The scripts should be run by :core:man1:`flux-cron`:
 
  30 * * * * bash -c "flux account-fetch-job-records; flux account update-usage; flux account-update-fshare; flux account-priority-update"
 
+Periodically fetching and storing job records in the flux-accounting database
+can cause the DB to grow large in size. Since there comes a point where job
+records become no longer useful to flux-accounting in terms of job usage and
+fair-share calculation, you can pass the ``--scrub`` optional argument to
+``flux account-fetch-job-records`` to also remove old job records. By default,
+passing ``--scrub`` will delete any job record that has completed more than
+6 months ago. This can be tuned by specifying the number of weeks to go back
+when determining which records to remove. The example below will fetch new job
+records and remove any job record more than 4 weeks old:
+
+.. code-block:: console
+
+ $ flux account-fetch-job-records --scrub 4
+
+By default, the memory occupied by a SQLite database does not decrease when
+records are `DELETE`d from the database. After scrubbing old job records from
+the flux-accounting database, if space is still an issue, the `VACUUM` command
+will clean up the space previously occupied by those deleted records. You can
+run this command by connecting to the flux-accounting database in a SQLite
+shell:
+
+.. code-block:: console
+
+ $ sqlite3 FluxAccounting.db
+ sqlite> VACUUM;
+
 ***********************
 Database Administration
 ***********************