Fix #6926 .

Hi,

Hi!

When invoking from_dlpack() and to_dlpack, the following warnings are displayed:

from_dlpack()

/opt/conda/envs/rapids/lib/python3.7/site-packages/cudf/io/dlpack.py:33: UserWarning: WARNING: cuDF from_dlpack() assumes column-major (Fortran order) input. If the input tensor is row-major, transpose it before passing it to this function.
res = libdlpack.from_dlpack(pycapsule_obj)
to_dlpack()

/opt/conda/envs/rapids/lib/python3.7/site-packages/cudf/io/dlpack.py:74: UserWarning: WARNING: cuDF to_dlpack() produces column-major (Fortran order) output. If the output tensor needs to be row major, transpose the output of this function.
return libdlpack.to_dlpack(gdf_cols)
I think those warnings should be removed, because it contains information that should be available in the API documentation, and not necessarily displayed each time the methods are invoked.

Some users, like me, love to have their notebooks/code without warnings. Even if it is possible to disable those warnings, I think the user should not go that way, because the warning is just repeating what the API documentation should cover.

Hope it helps!
Miguel

Moved API documentation from warning message to API doc.

Removed trailing space and reduced the line length to <=80

I have just moved the warning to a NOTES section.

Updated copyright notice from 2019 to 2019-2020.