[D1] Clarify sharing D1 across products locally (#18475)

* Adding a line to explain where a user can specify their custom migration dir.

* Adding a sentence on the use-case of migrations_dir.

* Update src/content/docs/workers/wrangler/configuration.mdx

Author: Oxyjun

src/content/docs/d1/reference/migrations.mdx

@@ -35,7 +35,7 @@ database_name = "<DATABASE_NAME>"
 database_id = "<UUID>"
 preview_database_id = "<UUID>"
 migrations_table = "<d1_migrations>" # Customize this value to change your applied migrations table name
-migrations_dir = "<FOLDER_NAME>" # Customize this value to rename the `migrations` folder
+migrations_dir = "<FOLDER_NAME>" # Specify your custom migration directory
 ```
 
 </WranglerConfig>

src/content/docs/workers/wrangler/configuration.mdx

@@ -475,7 +475,8 @@ To bind D1 databases to your Worker, assign an array of the below object to the
 
 - `migrations_dir` <Type text="string" /> <MetaInfo text="optional" />
 
-  - The migration directory containing the migration files. By default, `wrangler d1 migrations create` creates a folder named `migrations`. You can use `migrations_dir` to specify a different folder containing the migration files. For more information, refer to [D1 Wrangler `migrations` commands](/workers/wrangler/commands/#migrations-create) and [D1 migrations](/d1/reference/migrations/).
+  - The migration directory containing the migration files. By default, `wrangler d1 migrations create` creates a folder named `migrations`. You can use `migrations_dir` to specify a different folder containing the migration files (for example, if you have a mono-repo setup, and want to use a single D1 instance across your apps/packages).
+  - For more information, refer to [D1 Wrangler `migrations` commands](/workers/wrangler/commands/#migrations-create) and [D1 migrations](/d1/reference/migrations/).
 
 :::note