[docs] Placement group documentation (#10555)

Co-authored-by: Richard Liaw <rliaw@berkeley.edu>

python/ray/state.py

@@ -407,8 +407,15 @@ class GlobalState:
         def get_strategy(strategy):
             if strategy == PlacementStrategy.PACK:
                 return "PACK"
-            else:
+            elif strategy == PlacementStrategy.STRICT_PACK:
+                return "STRICT_PACK"
+            elif strategy == PlacementStrategy.STRICT_SPREAD:
+                return "STRICT_SPREAD"
+            elif strategy == PlacementStrategy.SPREAD:
                 return "SPREAD"
+            else:
+                raise ValueError(
+                    f"Invalid strategy returned: {PlacementStrategy}")
 
         assert placement_group_info is not None
         return {