docs(core): document projectsAffectedByDependencyUpdates (#27134)

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## Current Behavior
No docs for new config flag

## Expected Behavior
Docs 🎉 

## Related Issue(s)
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merged. -->

Fixes #22259

docs/shared/using-nx/affected.md

@@ -244,6 +244,22 @@ Nx provides two methods to exclude glob patterns (files and folders) from `affec
 - Glob patterns defined in your `.gitignore` file are ignored.
 - Glob patterns defined in an optional `.nxignore` file are ignored.
 
+## Marking Projects Affected by Dependency Updates
+
+By default, Nx will mark all projects as affected whenever your package manager's lock file changes. This behavior is a failsafe in case Nx misses a project that should be affected by a dependency update. If you'd like to opt in to a smarter behavior you can configure Nx to only mark projects as affected if they actually depend on the updated packages.
+
+```json {% fileName="nx.json" %}
+{
+  "pluginsConfig": {
+    "@nx/js": {
+      "projectsAffectedByDependencyUpdates": "auto"
+    }
+  }
+}
+```
+
+The flag `projectsAffectedByDependencyUpdates` can be set to `auto`, `all`, or an array that contains project specifiers. The default value is `all`.
+
 ## Not Using Git
 
 If you aren't using Git, you can pass `--files` to any affected command to indicate what files have been changed.