Fix docstring

Author: cphyc

yt/data_objects/selection_objects/data_selection_objects.py

@@ -536,9 +536,9 @@ def piter(
             from each processor.
             - None: no reduction will be applied and the storage object will
                 contain one result per chunk in the container.
-            - concat: the storage object will contain a flattened list of
-                each results.
-            - cat_on_root: same as concat, but only the root processor will
+            - cat: the storage object will contain a flattened list of
+                each result.
+            - cat_on_root: same as cat, but only the root processor will
                 contain anything.
             - sum, min, max: the storage object will contain the result
                 of applying the operation until getting a single value.

yt/utilities/parallel_tools/parallel_analysis_interface.py

@@ -517,9 +517,9 @@ def parallel_objects(
         from each processor.
         - None: no reduction will be applied and the storage object will
             contain one result per chunk in the container.
-        - concat: the storage object will contain a flattened list of
-            each results.
-        - cat_on_root: same as concat, but only the root processor will
+        - cat: the storage object will contain a flattened list of
+            each result.
+        - cat_on_root: same as cat, but only the root processor will
             contain anything.
         - sum, min, max: the storage object will contain the result
             of applying the operation until getting a single value.
@@ -1017,6 +1017,7 @@ def concat(
     ):
         """
         Concatenate all the values of data and send them to the root.
+        This returns None on all other processors.
 
         Notes:
         ------