added list of numpy functions known to drop units

requested on ticket #3035
This causes the sphinx build on travis-ci to fail:
https://travis-ci.org/astropy/astropy/jobs/38445646#L2434
```
/home/travis/build/astropy/astropy/docs/known_issues.rst:50: ERROR: Unknown target name: "np.c".
/home/travis/build/astropy/astropy/docs/known_issues.rst:50: ERROR: Unknown target name: "np.r".
```

Can you please fix this and use these commands locally to check that your list looks ok?
```
python setup.py build_sphinx
open docs/_build/html/index.html
```

For numpy we have intersphinx and you can create links by putting e.g.
```
`numpy.hstack`
```
instead of
```
np.hstack
```
For things where Sphinx interlink doesn't work you should put them in double-backticks to avoid the sphinx error.