SyncFlash: document :public ETS table option

lib/mv/vereinfacht/sync_flash.ex

@@ -35,6 +35,8 @@ defmodule Mv.Vereinfacht.SyncFlash do
 
   @doc false
   def create_table! do
+    # :public so any process can write (SyncContact runs in LiveView/Ash transaction process,
+    # not the process that created the table). :protected would restrict writes to the creating process.
     if :ets.whereis(@table) == :undefined do
       :ets.new(@table, [:set, :public, :named_table])
     end