doc: add housekeeping documentation to flux-config-job-manager(5)

Problem: The job-manager.housekeeping config table is not documented.

Document it in flux-config-job-manager(5).

doc/man5/flux-config-job-manager.rst

@@ -28,6 +28,20 @@ plugins
    Each directive follows the format defined in the :ref:`plugin_directive`
    section.
 
+housekeeping
+   (optional) Table of configuration for the job-manager housekeeping
+   service. The housekeeping service is an experimental alternative for
+   handling administrative job epilog workloads. If enabled, resources are
+   released by jobs to housekeeping, which runs a command or a systemd unit
+   and releases resources to the scheduler on completion. See configuration
+   details in the :ref:`housekeeping` section.
+
+   Note: The housekeeping script runs as the instance owner (e.g. "flux").
+   On a real system, "command" is be configured to "imp run housekeeping",
+   and the IMP is configured to launch the flux-housekeeping systemd
+   service as root. (See :man5:`flux-config-security-imp` for details
+   on configuring :command:`flux imp run`).
+
 
 .. _plugin_directive:
 
@@ -50,6 +64,28 @@ conf
    (optional) An object, valid with ``load`` only, that defines a configuration
    table to pass to the loaded plugin.
 
+.. _housekeeping:
+
+HOUSEKEEPING
+============
+
+command
+  (optional) An array of strings specifying the housekeeping command. Either
+  ``command`` or ``use-systemd-unit`` must be specified.
+
+use-systemd-unit
+  (optional) A boolean value indicating whether to run the flux-housekeeping
+  systemd unit to handle housekeeping, rather than a specific command.
+  Either ``use-systemd-unit`` or ``command`` must be specified.
+
+release-after
+  (optional) A string specified in Flux Standard Duration (FSD). If unset,
+  resources for a given job are not released until all execution targets for
+  a given job have completed housekeeping. If set to ``0``, resources are
+  released as each target completes. Otherwise, a timer is started when the
+  first execution target for a given job completes, and all resources that
+  have completed housekeeping when the timer fires are released. Following
+  that, resources are released as each execution target completes.
 
 EXAMPLE
 =======
@@ -73,6 +109,10 @@ EXAMPLE
       }
    ]
 
+   [job-manager.housekeeping]
+   use-systemd-unit = true
+   release-after = "1m"
+
 
 RESOURCES
 =========