docs: clarify server.deps.fallbackCJS heuristic (#9479)

Author: gilverse-icn

docs/config/server.md

@@ -68,6 +68,6 @@ If a `RegExp` is provided, it is matched against the full file path.
 - **Type:** `boolean`
 - **Default:** `false`
 
-When a dependency is a valid ESM package, try to guess the cjs version based on the path. This might be helpful, if a dependency has the wrong ESM file.
+When enabled, Vitest will try to guess a CommonJS build for an ESM entry by checking a few common CJS/UMD file name and folder patterns (like `.mjs`, `.umd.js`, `.cjs.js`, `umd/`, `cjs/`, `lib/`).
 
-This might potentially cause some misalignment if a package has different logic in ESM and CJS mode.
+This is a best-effort heuristic to work around confusing or incorrect ESM/CJS packaging and may not work for all dependencies.