DOC: Add warning message to np.npv docs

``np.npv`` calculates the present value of a series of cashflows
starting in the present (t=0). Another common definition of NPV
uses the future cashflows i.e. starting at the end of the first
period.

The difference in definition can lead to confusion for users moving
between NumPy and other projects [1] and adds complexity for
maintainers [2]. This commit adds a warning to the ``npv``
function's documentation and adds an example on how to find the npv
of a project using the alternate definition.

[1] https://github.com/numpy/numpy/issue/10877
[2] https://github.com/numpy/numpy/pull/3346

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