docs: clarify that the server must be run in development mode to generate openapi specs (#4634)

docs/docs/developer/open-api.md

@@ -4,7 +4,7 @@ Immich uses the [OpenAPI](https://swagger.io/specification/) standard to generat
 
 ## Generator
 
-OpenAPI is used to generate the client (Typescript, Dart) SDK. `openapi-generator-cli` can be installed [here](https://openapi-generator.tech/docs/installation/). The generated SDK is based on the `immich-openapi-specs.json` file, which is autogenerated by the server when running in development mode. The `immich-openapi-specs.json` file can be modified with `@nestjs/swagger` decorators used or referenced by controller endpoints. See the [NestJS OpenAPI docs](https://docs.nestjs.com/openapi/types-and-parameters) for more info. When you add a new endpoint or modify an existing one, you must run the command below to update the client SDK.
+OpenAPI is used to generate the client (Typescript, Dart) SDK. `openapi-generator-cli` can be installed [here](https://openapi-generator.tech/docs/installation/). The generated SDK is based on the `immich-openapi-specs.json` file, which is autogenerated by the server **when running in development mode**. The `immich-openapi-specs.json` file can be modified with `@nestjs/swagger` decorators used or referenced by controller endpoints. See the [NestJS OpenAPI docs](https://docs.nestjs.com/openapi/types-and-parameters) for more info. When you add a new endpoint or modify an existing one, you must run the server in development mode and run the command below to update the client SDK.
 
 ```bash
 npm run api:generate # Run from the `server/` directory