BUG: fixing ewma() for adjust=False and ignore_na=False

[seth-p]

The previous code in #7603 wasn't quite right for the case that both adjust=False and ignore_na=False. This PR fixes the handling of that case. Note that ignore_na=False behavior is new in v0.15.0, so this bug and its fix don't affect any prior behavior.

[jreback]

side issue can you add a couple of examples in the doc-string of ewma? Including one with adust
thxs

[seth-p]

Happy to give it a shot, though to be honest I'm not quite sure what to put there. Can you point me to some other doc strings that have such examples? I can add more verbose notes about adjust (which to my mind isn't documented very clearly) and ignore_na to _ewm_notes (which is used to construct the doc strings for all the ewm*() functions), though I don't see numerical examples being illuminating.

[jreback]

http://pandas.pydata.org/pandas-docs/stable/generated/pandas.DataFrame.xs.html?highlight=xs#pandas.DataFrame.xs

(just showing some examples, these are for users to copy-paste). if it doesn't add then its ok. but a bit more docs are always better.

[seth-p]

I don't think examples will add much, as it's just setting adjust=True or False and producing different numbers, but they won't be intuitive either way. I will augment _ewm_notes to give a better explanation of what adjust and ignore_na do, and will update this PR.

[seth-p]

I've updated _ewm_notes to give a much clearer description of adjust and ignore_na. Let me know what you think.

[seth-p]

Actually, let me make a slight edit...

[jreback]

that looks good. does this need to also be on the other ewma_* doc-strings?

(you can simply put this in a variable and use Appender to propgate this)

[seth-p]

I put it in _ewm_notes, which I think is incorporated into all the ewm* doc strings automatically. I'm making a slight edit...

[seth-p]

OK, I'm done editing.
Yep, all the ewm* functions begin with @Substitution(..., _ewm_notes), so these updated notes should be reflected in all the ewm* doc strings.

[seth-p]

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