DOC: clean up DataFrame.ewm
A few issues I've noticed:

1) It says

> When adjust is False, weighted averages are calculated recursively as:
> 
>     weighted_average[0] = arg[0]; weighted_average[i] = (1-alpha)*weighted_average[i-1] + alpha*arg[i].
> 

  but `arg` isn't defined

2) The phrase "When adjust is False, weighted averages are calculated recursively as:" appears as bold, but I don't think this was the intention

3) 
```
\text{for} halflife > 0
```
should be
```
\text{for } halflife > 0
```

4) In the "notes" section, ```alpha``` is used, while in the description of the parameters, it's
```
:math:`\alpha` 
```

5) The respective doc for Series could be updated, as it says
```
axis{0 or ‘index’, 1 or ‘columns’}, default 0
```