DOC: fixup formula for exponentially weighted mean with input vector of times?

[MarcoGorelli]

Location of the documentation

https://pandas.pydata.org/docs/user_guide/window.html#exponentially-weighted-window

Documentation problem

Currently, it reads

However, I don't understand why the denominator doesn't also sum over time, as is shown in the general formula at the beginning of the section:

Suggested fix for documentation

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I haven't done much windowing at all, so this may very well be me misunderstanding something

cc @mroeschke in #40099 , then if the timedelta is zero, then I think the current formula would give an expanding sum rather than an expanding mean

[mroeschke]

Thanks for the catch @MarcoGorelli! You are correct; there should be a summation in the denominator as well.

[mschmookler]

take