DOC: clarify pmt cash-flow signs

[naseweisssss]

Add some documentations in relation to issues #130 to clarify the usage of signs on fv and pv

[InessaPawson]

Thanks for working on this, @naseweisssss !
The zero rate example is a great idea, it's much easier to do the math. I checked against the implementation, and it's right.
A few small requests:

1. Keep "Future value" as the term for fv. That is the terminology used throughout the API: pv, nper, ipmt, ppmt, rate - all document the fv parameter as “Future value”. Also, fv itself is defined as computing the future value. Renaming it only in pmt would introduce an inconsistency.
2. Make the pv note perspective-independent, e.g., "Cash received is positive and cash paid out is negative. For a borrower, the loan amount is positive".
3. ipmt, ppmt, nper, and fv share the same convention and descriptions. A short note in the module docstring explaining the convention would be nice. Also could be a follow-up PR.