Add "known issue" for quantities sometimes losing their units
Clarify that issue propagates to more complex functions in numpy and scipy
Use single import in example (instead of two imports with a semicolon)
Follow PEP8 and split the import over two lines
And line numbers in example still makes sense
Merge pull request #1937 from mhvk/quantity-known-issues

Add "known issue" for quantities sometimes losing their units