Clarify allowed suffix for the log rotation

Closes #42068

Signed-off-by: Martin Bartoš <[email protected]>
(cherry picked from commit 7c39bb3)

Author: mabartos

core/runtime/src/main/java/io/quarkus/runtime/logging/FileConfig.java

@@ -2,6 +2,7 @@
 
 import java.io.File;
 import java.nio.charset.Charset;
+import java.time.format.DateTimeFormatter;
 import java.util.Optional;
 import java.util.logging.Level;
 
@@ -84,6 +85,8 @@ public static class RotationConfig {
          * The file handler rotation file suffix.
          * When used, the file will be rotated based on its suffix.
          * <p>
+         * The suffix must be in a date-time format that is understood by {@link DateTimeFormatter}.
+         * <p>
          * Example fileSuffix: .yyyy-MM-dd
          * <p>
          * Note: If the suffix ends with .zip or .gz, the rotation file will also be compressed.