Rent implementation is undocumented

[CriesofCarrots]

Problem

The only documentation we have on rent is here: https://docs.solana.com/implemented-proposals/rent, and it doesn't reflect eager rent-collection which landed last week.
It's also not very consumable for developers and other users who just want to know when and how and how much rent will be assessed to their accounts.

Proposed Solution

Add user-driven rent documentation