Update Documentation KDTreeFlann.search* functions

### Checklist

- [X] I have searched for [similar issues](https://github.com/isl-org/Open3D/issues).
- [X] For Python issues, I have tested with the [latest development wheel](http://www.open3d.org/docs/latest/getting_started.html#development-version-pip).
- [X] I have checked the [release documentation](http://www.open3d.org/docs/release/) and the [latest documentation](http://www.open3d.org/docs/latest/) (for `master` branch).

### Proposed new feature or change

Not a real feature rather a request for improving Documentation. Thus i checked latest Python documenation and Tutorials instead of wheel.
It occured to me now the second time that i wanted to use the distance returned by KDTreeFLANN and again i forgot that the Eigenbinder of nanoflann per default uses L2 norm as distance measure. Consequently the distances returned are `d^2` instead of `d`. This is a matter of speed optimization avoiding plenty of costly calls to `sqrt`.

So can you please when preparing the next release include in the KDTree documentation as well as all related KDTree tutorials (C/C++, Python etc.) include a description of L2 Norm and how to get the Euclidean distances from the list of distance measures returned by all search functions as last `open3d.utility.Vector3dVector`.

In Python for example it would be `np.sqrt(np.asarray(distances))`.

### References

http://www.open3d.org/docs/release/python_api/open3d.geometry.KDTreeFlann.html
https://jlblancoc.github.io/nanoflann/structnanoflann_1_1KDTreeEigenMatrixAdaptor.html

### Additional information

ps.: The FLANN link in the first paragraph of the Tutorial produces an Appache 500 i guess the linked site is not available any more.

**Edit'**
Sorry forgot to set appropriate title of this issue