Add documentation for the keep-alive flag

This commit updates the documentation of the task:executor element to
reference the keep-alive flag.

Issue: SPR-12407
(cherry picked from commit 6534d00)

Author: snicoll

src/asciidoc/index.adoc

@@ -44788,6 +44788,21 @@ element.
 			rejection-policy="CALLER_RUNS"/>
 ----
 
+Finally, the `keep-alive` setting determines the time limit (in seconds) for which threads
+may remain idle before being terminated. If there are more than the core number of threads
+currently in the pool, after waiting this amount of time without processing a task, excess
+threads will get terminated. A time value of zero will cause excess threads to terminate
+immediately after executing a task without remaining follow-up work in the task queue.
+
+[source,xml,indent=0]
+[subs="verbatim,quotes"]
+----
+	<task:executor
+			id="executorWithKeepAlive"
+			pool-size="5-25"
+			keep-alive="120"/>
+----
+
 
 
 [[scheduling-task-namespace-scheduled-tasks]]
@@ -45340,7 +45355,7 @@ message is surrounded by quotes. Below are the changes that I (the author) make
 
 		public String getMessage() {
 			// change the implementation to surround the message in quotes
-			return "'" + this.message + "'"
+			return "''" + this.message + "''"
 		}
 
 		public void setMessage(String message) {