update documentation

CONTRIBUTING.md

@@ -62,7 +62,7 @@ One notable change from the development workflow in other applications is code g
 
 While you have a backend development server running, run `yarn codegen` in a separate terminal. This process will watch the frontend and backend for changes to keep generated types up to date.
 
-(For CI, we also use `yarn codegen-once`. This will start a backend server, run code generation once, and exit.)
+(Alternatively, you can use `yarn codegen-once`. This will start a backend server, run code generation once, and close the server. This command only works on UNIX-based systems. Not that this does not work if a backend server is already running.)
 
 ### Recommended order of development