[docs] [issue33828] Add versionchanged notes for string.Formatter

[Xiang Zhang]

New submission from Xiang Zhang <angwerzx at 126.com>:

I propose to add versionchanged note about auto-numbering feature of string.Formatter, introduced in #13598. It's quite confusing which version could I use the feature reading the doc. Also it's better to note in 2.7 the feature is not available.

Currently the doc gives me the feeling string.Formatter and str.format share the same feature and the auto-numbering feature is available in 2.7 and >3.1.

----------
assignee: docs at python
components: Documentation
messages: 319284
nosy: docs at python, eric.smith, xiang.zhang
priority: normal
severity: normal
stage: needs patch
status: open
title: Add versionchanged notes for string.Formatter
versions: Python 3.6, Python 3.7, Python 3.8

_______________________________________
Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue33828>
_______________________________________