DOC: pandas/pandas/core/window/ewm.py
#### Location of the documentation
Docstring for `class EWM(_Rolling)` in ewm.py.

#### Documentation problem
The purposes of the `EWM` class is unclear because there's no documentation on what 'ewm' is abbreviating. One reason this is confusing is that `ewm` can easily be interpreted as either "exponentially weighted mean" or "exponentially weighted moving". The `EWM` class docstring actually muddies the water further by stating that the class provides "exponential weighted **(EW)** functions".

#### Suggested fix for documentation
If it's absolutely necessary to use an abbreviation like `ewm` then the docstring needs to clearly state what it's abbreviating. 