Improve documentation of `persistent_client_expiration` option.

Closes #2404. Thanks to Rainer Plischke.

ChangeLog.txt

@@ -9,6 +9,8 @@ Broker:
   MOSQ_EVT_MESSAGE event.
 - Fix unused flags in CONNECT command being forced to be 0, which is not
   required for MQTT v3.1. Closes #2522.
+- Improve documentation of `persistent_client_expiration` option.
+  Closes #2404.
 
 Client library:
 - Fix threads library detection on Windows under cmake. Bumps the minimum

man/mosquitto.conf.5.xml

@@ -842,15 +842,21 @@ log_timestamp_format %Y-%m-%dT%H:%M:%S
 			<varlistentry>
 				<term><option>persistent_client_expiration</option> <replaceable>duration</replaceable></term>
 				<listitem>
-					<para>This option allows persistent clients (those with
-						clean session set to false) to be removed if they do
-						not reconnect within a certain time frame. This is a
-						non-standard option. As far as the MQTT spec is
-						concerned, persistent clients persist forever.</para>
-					<para>Badly designed clients may set clean session to false
-						whilst using a randomly generated client id. This leads
-						to persistent clients that will never reconnect. This
-						option allows these clients to be removed.</para>
+					<para>
+						This option allows the session of persistent clients (those with clean
+						session set to false) <emphasis>that are not currently connected</emphasis> to be removed if they
+						do not reconnect within a certain time frame. This is a non-standard option
+						in MQTT v3.1. MQTT v3.1.1 and v5.0 allow brokers to remove client sessions.
+					</para>
+
+					<para>
+						Badly designed clients may set clean session to false whilst using a randomly
+						generated client id. This leads to persistent clients that connect once and
+						never reconnect. This option allows these clients to be removed.  This option
+						allows persistent clients (those with clean session set to false) to be
+						removed if they do not reconnect within a certain time frame.
+					</para>
+
 					<para>The expiration period should be an integer followed
 						by one of h d w m y for hour, day, week, month and year
 						respectively. For example:</para>

mosquitto.conf

@@ -141,14 +141,16 @@
 # accepted. MQTT imposes a maximum payload size of 268435455 bytes.
 #message_size_limit 0
 
-# This option allows persistent clients (those with clean session set to false)
-# to be removed if they do not reconnect within a certain time frame.
-#
-# This is a non-standard option in MQTT V3.1 but allowed in MQTT v3.1.1.
+# This option allows the session of persistent clients (those with clean
+# session set to false) that are not currently connected to be removed if they
+# do not reconnect within a certain time frame. This is a non-standard option
+# in MQTT v3.1. MQTT v3.1.1 and v5.0 allow brokers to remove client sessions.
 #
 # Badly designed clients may set clean session to false whilst using a randomly
-# generated client id. This leads to persistent clients that will never
-# reconnect. This option allows these clients to be removed.
+# generated client id. This leads to persistent clients that connect once and
+# never reconnect. This option allows these clients to be removed.  This option
+# allows persistent clients (those with clean session set to false) to be
+# removed if they do not reconnect within a certain time frame.
 #
 # The expiration period should be an integer followed by one of h d w m y for
 # hour, day, week, month and year respectively. For example