[Docs] Updating CMake docs to include LLVM_OPTIMIZED_TABLEGEN

This is based on feedback on llvm-commits from Sean Silvas.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@264318 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed