BUG: fixing ewma() for adjust=False and ignore_na=False
The previous code in https://github.com/pydata/pandas/pull/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.

side issue can you add a couple of examples in the doc-string of `ewma`? Including one with adust
thxs
 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.
 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.
 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.
 I've updated `_ewm_notes` to give a much clearer description of `adjust` and `ignore_na`. Let me know what you think.
 Actually, let me make a slight edit...
 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)
 I put it in `_ewm_notes`, which I think is incorporated into all the `ewm*` doc strings automatically. I'm making a slight edit...
 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.
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