Fix the javadoc for SyncFutureCache

/**
 * A cache of {@link SyncFuture}s. This class supports two methods
 * {@link SyncFutureCache#getIfPresentOrNew()} and {@link SyncFutureCache#offer()}.
 * <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.
 */