readyset-dataflow: Add docs comment for Domain

This adds a doc comment for the Domain struct, providing a simple
description of a Domain.

Change-Id: I0aa650bd430df3446b0b0aa751f216a8d746de75
Reviewed-on: https://gerrit.readyset.name/c/readyset/+/5269
Tested-by: Buildkite CI
Reviewed-by: Aspen Smith <[email protected]>

Author: Luke Osborne

readyset-dataflow/src/domain/mod.rs

@@ -577,6 +577,11 @@ impl RemappedKeys {
     }
 }
 
+/// A [`Domain`] is a well-connected sub-graph of the overall dataflow graph, used as the unit
+/// of execution of the dataflow engine. The dataflow graph is split up into domains using
+/// [heuristics](../src/readyset_server/controller/migrate/assignment.rs.html)
+/// Domains are [scheduled](../src/readyset_server/controller/migrate/scheduling.rs.html)
+/// onto workers.
 pub struct Domain {
     index: DomainIndex,
     shard: Option<usize>,