Document the new interface

docs/source/interface.rst

@@ -138,3 +138,19 @@ If it has run,
 
  * ``when``: associated with an ISO8601 datetime string giving the approximate time the process ran
  * ``count``: associated with a number giving the number of passes which would need to be spent to renew leases on all stored objects seen during the lease maintenance activity
+
+``POST /storage-plugins/privatestorageio/zkapauthz-v1/unblinded-token``
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+This endpoint allows an external agent to insert new unblinded tokens into the node's database.
+This allows for restoration of previously backed-up tokens in case the node is lost.
+Tokens inserted with this API will be used after any tokens already in the database and in the order they appear in the given list.
+
+The request body must be ``application/json`` encoded and contain an object like::
+
+  { "unblinded-tokens": [<unblinded token string>, ...]
+  }
+
+The response is **OK** with ``application/json`` content-type response body like::
+
+  { }