user doc: Clarify Unseen Only selection description based on Paul's comment
@@ -28,8 +28,8 @@ to obtain messages from a particular folder, enter the name of that folder.
+
If this connection accesses a POP3 email server, only standard folders,
such as *Inbox* or *Sent* are supported.
-.. Select the *Unseen Only* checkbox to obtain only messages that
-the connection has not previously returned.
+.. Select the *Unseen Only* checkbox to obtain
+only messages that are marked as unread on the email server.
.. In the *Delay* field, accept the default of 5 seconds as the time
that elapses between polls for messages. Or, to specify a different polling interval,
enter a number and select its time unit.
