[docs]: improve doc for @dev and difference of define doc string for better understanding

This suggestion comes from the review of https://github.com/Azure/typespec-azure/pull/4494

I asked our [chatbot](https://teams.microsoft.com/l/message/19:rMhMrxg7UjfwZmVoSeVvWvNQIfT_G6ds8napsytWqzw1@thread.tacv2/1779872767027?tenantId=72f988bf-86f1-41af-91ab-2d7cd011db47&groupId=39910aef-85da-4e30-b5e3-35f04ef38648&parentMessageId=1779872757921&teamName=Azure%20SDK%20Q%26A%20Bot%20Testing&channelName=Azure%20SDK%20QA%20bot%20for%20TypeSpec%20Testing&createdTime=1779872767027) and it provides a completely wrong answer because there no reference doc to describe

1. whether @dev is a built-in TypeSpec decorator
2. whether @doc("...") and /** ... */ doc comments are functionally equivalent

Need to improve the doc, otherwise seems no other people understand those concept.