DOC: fixup formula for exponentially weighted mean with input vector of times?
#### Location of the documentation

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

#### Documentation problem

Currently, it reads

![image](https://user-images.githubusercontent.com/33491632/110491485-3d5a5e80-80e9-11eb-90c9-07110438e26b.png)

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:

![image](https://user-images.githubusercontent.com/33491632/110493154-79da8a00-80ea-11eb-92b7-4de48b809a6a.png)

#### Suggested fix for documentation

![image](https://user-images.githubusercontent.com/33491632/110495689-a7c0ce00-80ec-11eb-96bd-f99dc8ba392f.png)

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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