chore: document multi-repo migrations (#4185)

Author: eakmanrq

docs/guides/multi_repo.md

@@ -184,6 +184,14 @@ gateways:
 
 Even if you do not have a need for multiple repos now, consider adding a `project` key so that you can easily support multiple repos in the future.
 
+## Running migrations with multiple repositories
+
+When doing a [migration](./migrations.md), pass in a single repo path using the `-p` flag. It doesn't matter which repo you choose.
+
+```
+$ sqlmesh -p examples/multi/repo_1 migrate
+```
+
 ## Multi-Repo dbt projects
 
 SQLMesh also supports multiple repos for dbt projects, allowing it to correctly detect changes and orchestrate backfills even when changes span multiple dbt projects.