Fix WriteTimeoutException java doc description (netty#9554)

Motivation:

The java doc doesn't match the real case: The exception only happen when a write operation
 cannot finish in a certain period of time instead of write idle happen.

Modification:

Correct java doc

Result:
java doc matched the real case

handler/src/main/java/io/netty/handler/timeout/WriteTimeoutException.java

@@ -18,8 +18,8 @@
 import io.netty.util.internal.PlatformDependent;
 
 /**
- * A {@link TimeoutException} raised by {@link WriteTimeoutHandler} when no data
- * was written within a certain period of time.
+ * A {@link TimeoutException} raised by {@link WriteTimeoutHandler} when a write operation
+ * cannot finish in a certain period of time.
  */
 public final class WriteTimeoutException extends TimeoutException {