doc: Add shouldExist for users

This addition to the docs was missed in the original PR
coreos#1014

Author: cgwalters

doc/configuration-v3_2_experimental.md

@@ -159,6 +159,7 @@ The Ignition configuration is a JSON document conforming to the following specif
     * **_noUserGroup_** (boolean): whether or not to create a group with the same name as the user. This only has an effect if the account doesn't exist yet.
     * **_noLogInit_** (boolean): whether or not to add the user to the lastlog and faillog databases. This only has an effect if the account doesn't exist yet.
     * **_shell_** (string): the login shell of the new account.
+    * **_shouldExist_** (boolean) If set to false, the target user will be deleted (if present) and not created.  Defaults to true.
     * **_system_** (bool): whether or not this account should be a system account. This only has an effect if the account doesn't exist yet.
   * **_groups_** (list of objects): the list of groups to be added. All groups must have a unique `name`.
     * **name** (string): the name of the group.