docs: minor updates to backend service doc

[melissahenderson]

I missed a few comments Sabine made on the sj-doc branch during the work week re: the backend service. This issue is to care for her missed comments.

• Change first bullet from "to perform account management tasks" to "payment initiation and information lookup"
• Third bullet re: "track balances" - Rafiki only tracks liquidity. The management of this is also done through the Admin API so maybe add it to the next bullet point.
• Requirements > third bullet (TigerBeetle) from "accounting balances at the ILP layer" to "accounting liquidity"
• Same bullet's note: From "accounting balances" to "accounting liquidities"