DOC: add pd.set_eng_float_format function in the API reference
- [x] closes #15610
- [N/A] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [N/A] whatsnew entry
Sorry my comment comes after you worked in the PR, but as I mentioned in the issue https://github.com/pandas-dev/pandas/issues/15610#issuecomment-413802674 I wouldn't merge this, as I think the right approach should be having this functionality with something like `pandas.set_option('display.eng_float_format')` Okay.. Can I close this PR? # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22395?src=pr&el=h1) Report
> Merging [#22395](https://codecov.io/gh/pandas-dev/pandas/pull/22395?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/84af863c38d305db96403f0afc5fa73b98052d0e?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/22395/graphs/tree.svg?width=650&src=pr&token=eZ4WkYLtcO&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/22395?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #22395   +/-   ##
=======================================
  Coverage   92.05%   92.05%           
=======================================
  Files         169      169           
  Lines       50709    50709           
=======================================
  Hits        46679    46679           
  Misses       4030     4030
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.46% <ø> (ø)` | :arrow_up: |
| #single | `42.25% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/22395?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/formats/format.py](https://codecov.io/gh/pandas-dev/pandas/pull/22395/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvZm9ybWF0LnB5) | `98.36% <ø> (ø)` | :arrow_up: |

------

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