[filesearch GPT] Documentation to inform the use of thread_id

use case- some orgs use contact variables to store the thread_ids, if the thread id is old (from a conversation before the unified api integration was done) then using in new flows would cause an error - 

https://discord.com/channels/717975833226248303/1467905945827938356/1480886750787276952

add documentation to inform NGOs to handle this case. 