Clarify ckpt configs

olmo/config.py

@@ -669,11 +669,29 @@ class ShardedCheckpointerType(StrEnum):
 
 class ActivationCheckpointingStrategy(StrEnum):
     whole_layer = "whole_layer"
+    """
+    Checkpoint every transformer layer.
+    """
+
     one_in_two = "one_in_two"
+    """
+    Checkpoint one in two transformer layers.
+    """
+
     one_in_three = "one_in_three"
+    """
+    Checkpoint one in three transformer layers.
+    """
+
     one_in_four = "one_in_four"
+    """
+    Checkpoint one in four transformer layers.
+    """    
+
     fine_grained = "fine_grained"
-
+    """
+    Focus checkpointing on where it is cheap to recompute and saves most memory.
+    """
 
 @dataclass
 class TrainConfig(BaseConfig):