Custom help for `vagrant gem` on top of RubyGems help

lib/vagrant/command/gem.rb

@@ -15,6 +15,14 @@ module Vagrant
           end
         end
 
+        # If the user needs some help, we add our own little message at the
+        # top so that they're aware of what `vagrant gem` is doing, really.
+        if @argv.empty? || @argv.include?("-h") || @argv.include?("--help")
+          @env.ui.info(I18n.t("vagrant.commands.gem.help_preamble"),
+                       :prefix => false)
+          puts
+        end
+
         # We just proxy the arguments onto a real RubyGems command
         # but change `GEM_HOME` so that the gems are installed into
         # our own private gem folder.

templates/locales/en.yml

@@ -231,6 +231,13 @@ en:
         vm_not_running: "VM is not currently running. Please bring it up to run this command."
       box:
         no_installed_boxes: "There are no installed boxes! Use `vagrant box add` to add some."
+      gem:
+        help_preamble: |-
+          `vagrant gem` is used to install Vagrant plugins via the RubyGems
+          system. In fact, `vagrant gem` is just a frontend to the actual `gem`
+          interface, with the difference being that Vagrant sets up a custom
+          directory where gems are installed so that they are isolated from your
+          system gems.
       init:
         success: |-
           A `Vagrantfile` has been placed in this directory. You are now