doc: Updates documentation for mongodbatlas_database_user resource to…

… guide users to set database name admin for custom roles (#1862)

website/docs/r/database_user.html.markdown

@@ -160,7 +160,7 @@ Block mapping a user's role to a database / collection. A role allows the user t
 ~> **IMPORTANT:** If a user is assigned a custom MongoDB role, they cannot be assigned any other roles.
 
 * `role_name` - (Required) Name of the role to grant. See [Create a Database User](https://docs.atlas.mongodb.com/reference/api/database-users-create-a-user/) `roles.roleName` for valid values and restrictions.
-* `database_name` - (Required) Database on which the user has the specified role. A role on the `admin` database can include privileges that apply to the other databases.
+* `database_name` - (Required) Database on which the user has the specified role. A role on the `admin` database can include privileges that apply to the other databases. This field should be set to `admin` for a custom MongoDB role.
 * `collection_name` - (Optional) Collection for which the role applies. You can specify a collection for the `read` and `readWrite` roles. If you do not specify a collection for `read` and `readWrite`, the role applies to all collections in the database (excluding some collections in the `system`. database).
 
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