docs: clarify cssHash in dev (#370)

Author: bluwy

docs/config.md

@@ -83,7 +83,7 @@ These options are specific to the Svelte compiler and are generally shared acros
 
 - **Type:** `CompileOptions` - See [svelte.compile](https://svelte.dev/docs#svelte_compile)
 
-  The options to be passed to the Svelte compiler. A few options are set by default, including `dev` and `css`. However, some options are non-configurable, like `filename`, `format`, `generate`, and `cssHash` (in dev).
+  The options to be passed to the Svelte compiler. A few options are set by default, including `dev` and `css`. However, some options are non-configurable, like `filename`, `format`, `generate`, and `cssHash` ([in dev](./faq.md#why-cant-csshash-be-set-in-development-mode)).
 
 ### preprocess
 

docs/faq.md

@@ -73,6 +73,12 @@ Bad:
 </style>
 ```
 
+### Why can't `cssHash` be set in development mode?
+
+`cssHash` is fixed in development for CSS HMR in Svelte components, ensuring that the hash value is stable based on the file name so that styles are only updated when changed.
+
+However, `cssHash` is respected in production builds as HMR is a dev-only feature.
+
 ### How do I add a Svelte preprocessor from a Vite plugin?
 
 If you are building a Vite plugin that transforms CSS or JS, you can add a `api.sveltePreprocess: PreprocessorGroup` to your Vite plugin definition and it will be added to the list of Svelte preprocessors used at runtime.