Help: Improve list command signature group name for read operations

The LENGTH, GET, JOIN, and SUBLIST operations all read the list without
modifying it.  Name their section appropriately.

Help/command/list.rst

@@ -33,8 +33,8 @@ scope.  To propagate the results of these operations upwards, use
   Be careful when counting with negative indices: they do not start from
   0.  -0 is equivalent to 0, the first list element.
 
-Capacity and Element access
-^^^^^^^^^^^^^^^^^^^^^^^^^^^
+Reading
+^^^^^^^
 
 LENGTH
 """"""