docs: expand description of externalConditions (#18619)

Co-authored-by: Bjorn Lu <bjornlu.dev@gmail.com>

Author: sapphi-red

Diff for: docs/config/ssr-options.md

@@ -44,4 +44,12 @@ These conditions are used in the plugin pipeline, and only affect non-externaliz
 - **Type:** `string[]`
 - **Default:** `['node']`
 
-Conditions that are used during ssr import (including `ssrLoadModule`) of externalized dependencies.
+Conditions that are used during ssr import (including `ssrLoadModule`) of externalized direct dependencies (external dependencies imported by Vite).
+
+:::tip
+
+When using this option, make sure to run Node with [`--conditions` flag](https://nodejs.org/docs/latest/api/cli.html#-c-condition---conditionscondition) with the same values in both dev and build to get a consistent behavior.
+
+For example, when setting `['node', 'custom']`, you should run `NODE_OPTIONS='--conditions custom' vite` in dev and `NODE_OPTIONS="--conditions custom" node ./dist/server.js` after build.
+
+:::