[clang] [UserManual] Document clang option for loop interchange (PR #147014)

[Madhur Amilkanthwar via cfe-commits]

madhur13490 wrote:

I see. I can drop this, but I couldn't find a link from the User Manual to the [Clang command line argument reference](https://clang.llvm.org/docs/ClangCommandLineReference.html#cmdoption-clang-floop-interchange). Is there one?

https://github.com/llvm/llvm-project/pull/147014