[CUDA] Call it "CUDA", not "CUDA C/C++" in our docs.

CUDA is an extension to C++ -- there is no such thing as "CUDA C".  But
also, the language is much more commonly called "CUDA" than "CUDA C++".

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@280849 91177308-0d34-0410-b5e6-96231b3b80d8
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