Help: Document CMAKE_TLS_VERIFY variable explicitly

Global setting for TLS_VERIFY should be encouraged. It seems desirable for similar reasons as UPDATE_DISCONNECTED global for FetchContent and ExternalProject.

I may not have all the wording/policies correct, but at least this prompts getting this variable called out more prominently.

CMAKE_TLS_VERIFY seems like something that should be encouraged akin to how EP_UPDATE_DISCONNECTED is encourages and similarly for FetchContent, to allow a user to globally configure. Since TLS verification can be an important part of cybersecurity, it seems better to not have to worry about commands missing this parameter--just set it once. In case of suspected broken certificates, verification can then be easily switched off temporarily.