Allow accounts to be re-activated from the admin APIs. (#7847)

docs/admin_api/user_admin_api.rst

@@ -91,10 +91,14 @@ Body parameters:
 
 - ``admin``, optional, defaults to ``false``.
 
-- ``deactivated``, optional, defaults to ``false``.
+- ``deactivated``, optional. If unspecified, deactivation state will be left
+  unchanged on existing accounts and set to ``false`` for new accounts.
 
 If the user already exists then optional parameters default to the current value.
 
+In order to re-activate an account ``deactivated`` must be set to ``false``. If
+users do not login via single-sign-on, a new ``password`` must be provided.
+
 List Accounts
 =============