Remove incorrect comment about checking generated C files in

[williamstein]

Evidently the decision was made at some point not to do what Dag suggests in the documentation of this file. So delete that suggestion, which conflicts with reality. I agree with not checking in the C files, and think Cython isn't an unreasonable dependency for building from git. Also, cython is clearly stated to be a build dependency here: https://github.com/numpy/numpy/blob/main/INSTALL.rst#id9

[williamstein]

This is only a change to a docstring, so should be very safe.

[mattip]

Thanks @williamstein

[InessaPawson]

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