fix(db-mongodb): add req to migration templates for transactions (#8407)

- Add `req` to MigrateUpArgs and MigrateDownArgs for mongodb
- Improve docs for transactions and migrations

packages/db-mongodb/src/createMigration.ts

@@ -11,11 +11,11 @@ const migrationTemplate = ({ downSQL, imports, upSQL }: MigrationTemplateArgs):
 } from '@payloadcms/db-mongodb'
 ${imports}
 
-export async function up({ payload }: MigrateUpArgs): Promise<void> {
+export async function up({ payload, req }: MigrateUpArgs): Promise<void> {
 ${upSQL ?? `  // Migration code`}
 }
 
-export async function down({ payload }: MigrateDownArgs): Promise<void> {
+export async function down({ payload, req }: MigrateDownArgs): Promise<void> {
 ${downSQL ?? `  // Migration code`}
 }
 `

packages/db-mongodb/src/types.ts

@@ -20,6 +20,7 @@ import type {
   JSONField,
   NumberField,
   Payload,
+  PayloadRequest,
   PointField,
   RadioField,
   RelationshipField,
@@ -109,5 +110,5 @@ export type FieldToSchemaMap<TSchema> = {
   upload: FieldGeneratorFunction<TSchema, UploadField>
 }
 
-export type MigrateUpArgs = { payload: Payload }
-export type MigrateDownArgs = { payload: Payload }
+export type MigrateUpArgs = { payload: Payload; req: PayloadRequest }
+export type MigrateDownArgs = { payload: Payload; req: PayloadRequest }