docs: clarify treeshake option behavior

Explain that the Rollup pass only post-processes esbuild's output for
extra tree shaking rather than replacing esbuild for the whole build,
and that treeshake: false simply skips that pass since esbuild's own
tree shaking always runs and can't be disabled.

Author: poetryofcode

docs/README.md

@@ -501,6 +501,8 @@ export default defineConfig({
 
 This option has the same type as the `treeshake` option in Rollup, [see more](https://rollupjs.org/guide/en/#treeshake).
 
+Note that this Rollup pass only post-processes the code esbuild already emitted to remove additional dead code — esbuild is still used to bundle the rest of the build. Setting `treeshake` to `false` (the default) simply skips this extra Rollup pass; esbuild's own built-in tree shaking still runs regardless of this option, since it can't be disabled.
+
 ### What about type checking?
 
 esbuild is fast because it doesn't perform any type checking, you already get type checking from your IDE like VS Code or WebStorm.