DOC: Fix misleading description of random_sample in numpy.random docstring.

The `random_sample` description seen at the top of the `help(numpy.random)` output is misleading:  it suggests that it's possible for the user to define the range of the uniform distribution.  This PR fixes that, and makes it clearer that `random` and `random_sample` are aliases for the same function.

Fixes #3920.