milestoneType: Clarify 'financing' code description

[jpmckinney]

As described in #438 (comment), the code is ambiguous, and can be interpreted as being appropriate for bill payments. The code description should be updated to reflect that it is for the financing of the procurement/contracting process, not for payment.

Since "financing" might not be correctly interpreted by non-native English speakers, we should expand on what this concept means more specifically, e.g. with a brief example.