Clarify quiet-down -block help text regarding Pipeline jobs (#27556)

The -block option of the quiet-down CLI command does not wait for
running Pipeline jobs, because they are resumable and pause at the end
of their current step. The usage text did not mention this, which led
users to report the behavior as a bug. Clarify the help text.

core/src/main/java/hudson/cli/QuietDownCommand.java

@@ -40,7 +40,7 @@ public class QuietDownCommand extends CLICommand {
 
     private static final Logger LOGGER = Logger.getLogger(QuietDownCommand.class.getName());
 
-    @Option(name = "-block", usage = "Block until the system really quiets down and no builds are running")
+    @Option(name = "-block", usage = "Block until the system really quiets down and no builds are running. Note that running Pipeline jobs do not block the quiet down: they pause at the end of their current step")
     public boolean block = false;
 
     @Option(name = "-timeout", usage = "If non-zero, only block up to the specified number of milliseconds")