added list of numpy functions known to drop units

requested on ticket #3035
@bnordgren - Could you please put those in single-backticks?
E.g.
```
* `numpy.dot`
```
This way Sphinx will generate a link to the numpy docs page for that function.

It would be best if you use these commands to check locally that the Sphinx build works and the output looks OK:
```
python setup.py build_sphinx
open docs/_build/html/index.html
```
Iterating via travis-ci to check things is slow and annoying for you, so that would be my recommendation to get this PR in shape quickly.