Explain the rationale behind 18 decimals format for RPS and snapshot amount

[smol-ninja]

The "About Precision" section has been removed from the Flow repo in sablier-labs/flow#431. However, some of its content can be useful for developers into understanding why we use 18 decimals format for RPS and snapshot amount.

Therefore, we should document it somewhere in the Sablier docs (TBD), answering the following questions:

1. Why does RPS use 18 decimals format instead of token's decimals?
2. Why is scaling and descaling of amounts necessary in deposit and withdraw?

References

1. About Precision section
2. Why are we still keeping diagrams in this repository? flow#412

[smol-ninja]

@andreivladbrg do we still need to add this to the docs? Or can we ask developers to refer to the docs in the contracts?