docs: add process.env.NODE_ENV inject tip (#3744)

Co-authored-by: neverland <[email protected]>

Author: 9aoy

website/docs/en/guide/advanced/env-vars.mdx

@@ -20,7 +20,7 @@ Rsbuild by default injects the some env variables into the code using [source.de
 
 - [process.env.BASE_URL](#processenvbase_url)
 - [process.env.ASSET_PREFIX](#processenvasset_prefix)
-- [process.env.NODE_ENV](#processenvnode_env)
+- [process.env.NODE_ENV](#processenvnode_env): This variable is injected by Rspack. For details, please refer to [Rspack - optimization.nodeEnv](https://rspack.dev/config/optimization#optimizationnodeenv))
 
 ### import.meta.env.MODE
 

website/docs/zh/guide/advanced/env-vars.mdx

@@ -20,7 +20,7 @@ Rsbuild 默认通过 [source.define](#使用-define) 向代码中注入以下环
 
 - [process.env.BASE_URL](#processenvbase_url)
 - [process.env.ASSET_PREFIX](#processenvasset_prefix)
-- [process.env.NODE_ENV](#processenvnode_env)
+- [process.env.NODE_ENV](#processenvnode_env)：该变量由 Rspack 注入，如需关闭或修改，可参考 [Rspack - optimization.nodeEnv](https://rspack.dev/zh/config/optimization#optimizationnodeenv)
 
 ### import.meta.env.MODE