[plugin] update NPX docs (#6849)

plugins/npx/README.md

@@ -11,7 +11,21 @@ This plugin automatically registers npx command-not-found handler if `npx` exist
 plugins=(.... npx)
 ```
 
-- Globally install npx binary (you need node.js installed too!)
+- Globally install npx binary (npx will be auto installed with recent versions of Node.js)
 ```bash
 sudo npm install -g npx
 ```
+
+## Note
+
+The shell auto-fallback doesn't auto-install plain packages. In order to get it to install something, you need to add `@`:
+
+```
+➜  jasmine@latest # or just `jasmine@`
+npx: installed 13 in 1.896s
+Randomized with seed 54385
+Started
+```
+
+It does it this way so folks using the fallback don't accidentally try to install regular typoes.
+