[add] document aboud random sampling
Random Sampling (``cupy.random``)
=================================
-CuPy's random number generation routines are based on cuRAND.
-They cover a small fraction of :mod:`numpy.random`.
-
The big difference of :mod:`cupy.random` from :mod:`numpy.random` is that :mod:`cupy.random` supports ``dtype`` option for most functions.
This option enables us to generate float32 values directly without any space overhead.
+When major update of CuPy, numbers generated by `cupy.random` of new version
+may not be same as that of previous version even if the same seed and
+distribution is used. However this does not mean distribution from which
+numbers is generated is wrong.
+
Sample random data
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