[Doc]: Need to improve the content for ARM Auto Signoff

when testing the answer from chat bot, looks the bot does not understand the content in
https://github.com/Azure/azure-rest-api-specs/wiki/ARM-Auto-SignOff

I think this spec need to be updated to better describe 'PR auto signoff.'

Start by reading the ARM Auto SignOff wiki page linked in the issue and compare its explanation with the chatbot's misunderstanding. Update the page so the PR auto signoff process is described more clearly, with completion judged by whether the relevant content is understandable to the chatbot and readers.

Written by the indexing model from the issue text.