docs/source/features/efel_features.rst documentation needs update

[anilbey]

Hi,

I wanted to inform that eFEL features received some updates since this file is created.
The recent features can be found here: https://efel.readthedocs.io/

uncertainpy/docs/source/features/efel_features.rst

Line 15 in ffb2400

eFEL currently contains 153 different features, we briefly list