docs: update docstrings

Author: Fabio Ferreira

monai/networks/nets/unet.py

@@ -27,6 +27,7 @@
 
 
 class _ActivationCheckpointWrapper(nn.Module):
+    """Apply activation checkpointing to the wrapped module during training."""
     def __init__(self, module: nn.Module) -> None:
         super().__init__()
         self.module = module
@@ -86,6 +87,8 @@ class UNet(nn.Module):
             if a conv layer is directly followed by a batch norm layer, bias should be False.
         adn_ordering: a string representing the ordering of activation (A), normalization (N), and dropout (D).
             Defaults to "NDA". See also: :py:class:`monai.networks.blocks.ADN`.
+        use_checkpointing: if True, apply activation checkpointing to internal sub-blocks during training to reduce memory 
+        at the cost of extra compute. Checkpointing is bypassed in eval and when gradients are disabled. Defaults to False.
 
     Examples::