[Docs] Add warning about using application code in block migrations (#…

…1475)

<img width="968" alt="image"
src="https://github.com/vivid-planet/comet/assets/48853629/874f0fcc-cab7-4ad0-b42c-5c2d396892b5">

docs/docs/blocks/block-migrations.mdx

@@ -126,6 +126,11 @@ export class ChangeEyebrowMigration extends BlockMigration<(from: From) => To> i
 }
 ```
 
+:::caution
+Avoid using application code in the migration.
+Doing so prevents the need to adapt old migrations if the code changes at a later time.
+:::
+
 Finally, we add the migration to the block using the `migrate` option in `createBlock`.
 
 ```ts