bpo-42385: [Enum] add `_generate_next_value_` to StrEnum (GH-23735)

The default for auto() is to return an integer, which doesn't work for `StrEnum`.  The new `_generate_next_value_` for `StrEnum` returns the member name, lower cased.
diff --git a/Doc/library/enum.rst b/Doc/library/enum.rst
index a9584b9..a0b078c 100644
--- a/Doc/library/enum.rst
+++ b/Doc/library/enum.rst
@@ -67,10 +67,12 @@
 
 .. class:: auto
 
-    Instances are replaced with an appropriate value for Enum members.  By default, the initial value starts at 1.
+    Instances are replaced with an appropriate value for Enum members.
+    :class:`StrEnum` defaults to the lower-cased version of the member name,
+    while other Enums default to 1 and increase from there.
 
 .. versionadded:: 3.6  ``Flag``, ``IntFlag``, ``auto``
-
+.. versionadded:: 3.10  ``StrEnum``
 
 Creating an Enum
 ----------------