HBASE-27906 Fix the javadoc for SyncFutureCache (apache#5325)

(cherry picked from commit 2c92e6f)
(cherry picked from commit a0c5c8e)
Change-Id: Icc4ae5282b1efc463f788c13eb88eed843a465f8

hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/wal/SyncFutureCache.java

@@ -27,12 +27,20 @@
 
 /**
  * A cache of {@link SyncFuture}s. This class supports two methods
- * {@link SyncFutureCache#getIfPresentOrNew()} and {@link SyncFutureCache#offer(SyncFuture)}. Usage
- * pattern: SyncFuture sf = syncFutureCache.getIfPresentOrNew(); sf.reset(...); // Use the sync
- * future finally: syncFutureCache.offer(sf); Offering the sync future back to the cache makes it
- * eligible for reuse within the same thread context. Cache keyed by the accessing thread instance
- * and automatically invalidated if it remains unused for
- * {@link SyncFutureCache#SYNC_FUTURE_INVALIDATION_TIMEOUT_MINS} minutes.
+ * {@link SyncFutureCache#getIfPresentOrNew()} and {@link SyncFutureCache#offer(SyncFuture)}}.
+ * <p>
+ * Usage pattern:
+ *
+ * <pre>
+ *   SyncFuture sf = syncFutureCache.getIfPresentOrNew();
+ *   sf.reset(...);
+ *   // Use the sync future
+ *   finally: syncFutureCache.offer(sf);
+ * </pre>
+ *
+ * Offering the sync future back to the cache makes it eligible for reuse within the same thread
+ * context. Cache keyed by the accessing thread instance and automatically invalidated if it remains
+ * unused for {@link SyncFutureCache#SYNC_FUTURE_INVALIDATION_TIMEOUT_MINS} minutes.
  */
 @InterfaceAudience.Private
 public final class SyncFutureCache {