DOC: ewm constructor
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit) for how to run them


* Made language more consistent in the Parameters
* Organized and added more Examples

@mroeschke I would suggest to use this as an opportunity to add an emphasis in the documentation about the difference in behaviour between `.rolling()` and `.ewm()` in their use of the `min_periods` argument default, this is something that has caught me out before:
````python
import pandas as pd

df = pd.DataFrame(range(10), columns=["spot"])

df["rolling"] = df["spot"].rolling(5).mean()
df["ewm"] = df["spot"].ewm(5).mean()
df
````
Output:
````console
   spot  rolling       ewm
0     0      NaN  0.000000
1     1      NaN  0.545455
2     2      NaN  1.120879
3     3      NaN  1.725782
4     4      2.0  2.359493
5     5      3.0  3.021172
6     6      4.0  3.709837
7     7      5.0  4.424377
8     8      6.0  5.163576
9     9      7.0  5.926138
```` @sdementen sure would take a PR for that. I think this should be in the docs themselves with references (but wouldn't object to also in the doc-strings). > @sdementen sure would take a PR for that. I think this should be in the docs themselves with references (but wouldn't object to also in the doc-strings).

@jreback for what would you take a PR? I am bit lost in the conversation as I can't find back the context from this PR sorry meant to tag @sdfordham 