added list of numpy functions known to drop units

requested on ticket #3035
Short answer: no, I can't run those commands locally. Your build environment appears to be incompatible with the Enthought Canopy distribution for Windows. I'll push the commit with the single backticks, but I'm not interested in setting up an entire build environment just to get a couple of lines of documentation added. At this point, the developers have my blessing to take over any residual formatting issues. I'm sure this is annoying for you too.

This would be a good thing to keep in mind the next time a developer suggests that a user edit documentation using github's web interface: creating a pull request that way does not imply a functioning build environment, or interest in making one.

```
(Canopy 64bit) C:\Users\bnordgren\Documents\Orchidee\src\astropy>python setup.py
 build_sphinx
The requested path 'astropy_helpers' for importing astropy_helpers does not exist, 
or does not contain a copy of the astropy_helpers package.  Attempting download instead.
Downloading astropy_helpers; run setup.py with the --offline option to force off
line installation.
Traceback (most recent call last):
  File "setup.py", line 21, in <module>
    from astropy_helpers.distutils_helpers import is_distutils_display_option
ImportError: No module named distutils_helpers

(Canopy 64bit) C:\Users\bnordgren\Documents\Orchidee\src\astropy>
```