ISPMT: "amortized investment" is jargon

Use the term "amortized loan" instead of "amortized investment". I don't think we can avoid using the term "amortized", since that is a term of art with significance. It refers to a fixed-term, loan which is paid off at equal intervals, with equal payments. A portion of the payment goes to accrued interest and the remainder pays down the principle. If we want to have a definitions section at the start of the financial functions, we can add that. Also, the note "Note that Currency is a subtype of Number; it is shown as Currency since many implementations track this subtype and will set displays accordingly." does not belong here. It is already defined in 3.3.5.