docs: document define behavior change

Author: yyx990803

docs/config/index.md

@@ -105,7 +105,11 @@ export default ({ command, mode }) => {
 
 - **Type:** `Record<string, string>`
 
-  Define global variable replacements. Entries will be defined as globals during dev and statically replaced during build. Replacements are performed only when the match is surrounded by word boundaries (`\b`).
+  Define global variable replacements. Entries will be defined as globals during dev and statically replaced during build.
+
+  - Starting from `2.0.0-beta.70`, string values will be used as raw expressions, so if defining a string constant, it needs to be explicitly quoted (e.g. with `JSON.stringify`).
+
+  - Replacements are performed only when the match is surrounded by word boundaries (`\b`).
 
 ### plugins