Make the coding standards a bit more clear that we prefer the fancy new

Make the coding standards a bit more clear that we prefer the fancy new
auto-brief format for doxygen comments. Most notable is switching to
that in the example doxygen comment. I've also tweaked the wording but
am happy to tweak it further if others have suggestions here.

Mostly doing this to capture something I and others have been writing
consistently and repeatedly in code reviews.

llvm-svn: 280419