INT-3861: Mail Namespace Support for userFlag

JIRA: https://jira.spring.io/browse/INT-3861

Allow the custom `userFlag` to be configured using the namespace.

Polishing according PR comments

src/reference/asciidoc/mail.adoc

@@ -271,7 +271,7 @@ As discussed in <<search-term>>, the default `SearchTermStrategy` will ignore me
 
 Starting with _version 4.2.2_, the name of the user flag can be set using `setUserFlag` on the `MailReceiver` - this
 allows multiple receivers to use a different flag (as long as the mail server supports user flags).
-Changing the user flag is not currently supported by the XML namespace.
+The attribute `user-flag` is available when configuring the adapter with the namespace.
 
 [[mail-filtering]]
 === Email Message Filtering

src/reference/asciidoc/whats-new.adoc

@@ -20,3 +20,9 @@ It was completely ignored; the gateway's reply goes to the next chain element, o
 if the gateway is the last element.
 This condition is now detected and disallowed.
 If you have such configuration, simply remove the `reply-channel`.
+
+==== Mail Changes
+
+The customizable `userFlag` added in 4.2.2 to provide customization of the flag used to denote that the mail has been
+seen is now available using the XML namespace.
+See <<imap-seen>> for more information.