#7175 improve vagrant readme (#7247)

plugins/vagrant/README.md

 # Vagrant plugin
 
-This plugin adds completion for [Vagrant](https://www.vagrantup.com/)
+This plugin adds autocompletion for [Vagrant](https://www.vagrantup.com/) commands, task names, box names and built-in handy documentation.
 
 To use it, add `vagrant` to the plugins array in your zshrc file:
 
 ```zsh
 plugins=(... vagrant)
 ```
+