Add prune command to documentation

website/source/docs/boxes/versioning.html.md

@@ -103,3 +103,5 @@ if they might be in use by other Vagrant environments. Because boxes can
 be large, you may want to actively prune them once in a while using
 `vagrant box remove`. You can see all the boxes that are installed
 using `vagrant box list`.
+
+Another option is to use `vagrant box prune` command to remove all installed boxes that are outdated and not currently in use.

website/source/docs/cli/box.html.md

@@ -150,6 +150,24 @@ with the `--all` flag.
   name. This is only required if a box is backed by multiple providers.
   If there is only a single provider, Vagrant will default to removing it.
 
+
+# Box prune
+
+**Command: `vagrant box prune`**
+
+This command removes old versions of installed boxes. If the box in currently in use vagrant will ask you if you to confirm.
+
+## Options
+
+* `--provider PROVIDER` - The specific provider type for the boxes to destroy.
+
+* `--dry-run` - Only print the boxes that would be removed.
+
+* `--name NAME` - The specific box name to check for outdated versions.
+
+* `--force` - Destroy without confirmation even when box is in use.
+
+
 # Box Repackage
 
 **Command: `vagrant box repackage NAME PROVIDER VERSION`**