Make pd.set_eng_float_format() into proper pandas.options.display option
`set_eng_float_format` is currently a top-level function and not an option. We should add a new option to `pandas.options.display` for this, and then potentially deprecate the top-level function.

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#### Problem description

The top-level function `set_eng_float_format()` is only documented through an example here http://pandas.pydata.org/pandas-docs/stable/options.html#number-formatting

but there is no sign of its existence in the API reference http://pandas.pydata.org/pandas-docs/stable/api.html

#### Proposed Solution

Add `set_eng_float_format()` in the following section: http://pandas.pydata.org/pandas-docs/stable/api.html#general-utility-functions

The implementation is currently [here](https://github.com/pandas-dev/pandas/blob/648ae4f03622d8eafe1ca3b833bd6a99f56bece4/pandas/formats/format.py#L2758)