Fix incorrect defaults for use_combined_linear and add default for use_flash_attention in docstring

Author: eitag-uni

monai/networks/nets/diffusion_model_unet.py

@@ -1527,9 +1527,9 @@ class DiffusionModelUNet(nn.Module):
         upcast_attention: if True, upcast attention operations to full precision.
         dropout_cattn: if different from zero, this will be the dropout value for the cross-attention layers.
         include_fc: whether to include the final linear layer. Default to True.
-        use_combined_linear: whether to use a single linear layer for qkv projection, default to True.
+        use_combined_linear: whether to use a single linear layer for qkv projection, default to False.
         use_flash_attention: if True, use Pytorch's inbuilt flash attention for a memory efficient attention mechanism
-            (see https://pytorch.org/docs/2.2/generated/torch.nn.functional.scaled_dot_product_attention.html).
+            (see https://pytorch.org/docs/2.2/generated/torch.nn.functional.scaled_dot_product_attention.html), default to False.
     """
 
     def __init__(