Document service buffer size

[oxarbitrage]

In #2718 we created a new service with a buffer size of 20 (

zebra/zebrad/src/commands/start.rs

Line 73 in fd418b3

let mempool = ServiceBuilder::new().buffer(20).service(mempool_service);

) because the state service uses the same size (

zebra/zebrad/src/commands/start.rs

Line 60 in fd418b3

let state = ServiceBuilder::new().buffer(20).service(state_service);

).

We are not sure if this is the right thing to do so we should find out why 20 is a good choice and document this in the code.

[teor2345]

This seems like it's not that important.