Correct javadoc to be consistent with the current behavior (#12152)

Motivation:

4797b88 change that default value related to ThreadLocal caches of buffers but did miss to update the javadocs

Modifications:

Correct javadocs

Result:

Javadocs reflect reality

buffer/src/main/java/io/netty/buffer/PooledByteBufAllocator.java

@@ -430,7 +430,7 @@ public static int defaultMaxOrder() {
     }
 
     /**
-     * Default thread caching behavior - System Property: io.netty.allocator.useCacheForAllThreads - default true
+     * Default thread caching behavior - System Property: io.netty.allocator.useCacheForAllThreads - default false
      */
     public static boolean defaultUseCacheForAllThreads() {
         return DEFAULT_USE_CACHE_FOR_ALL_THREADS;