gh-89867: string.Formatter auto numbering doc updates

[dg-pb]

• Issue: string.Formatter.parse does not handle auto-numbered positional fields #89867

[dg-pb]

@ZeroIntensity any chance you could check this one?

[ZeroIntensity]

I'm not enough of an expert here to review, but the English and markup look mostly fine to me (see my one comment). I think @picnixz is the right person to tag, but he's going to be out for a little while last I heard.

[dg-pb]

Thanks, english check is what I hoped for.

Regarding logic, maybe @encukou could have a look? This is just clarifying docs after #21767.

[encukou]

LGTM, thank you!