[Docs] Some updates to the Clang user's manual (#151702)
* Fills out the terminology section
* Removes the basic usage section (we should bring it back someday
though!)
* Updates the list of supported language versions
* Adds information about what versions of Clang are officially supported
* Moves some extensions into the intentionally unsupported extensions
section.
There are likely far more updates that could be done, but this seemed
worth posting just to get things moving.
