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

[Ryotaro Kasuga via cfe-commits]

https://github.com/kasuga-fj commented:

It is already documented in [Clang command line argument reference](https://clang.llvm.org/docs/ClangCommandLineReference.html#cmdoption-clang-floop-interchange) (perhaps this is automatically generated based on [`clang/Driver/Options.td`](https://github.com/llvm/llvm-project/blob/81e6552a3d6835c4e10eb981402febfac9df6156/clang/include/clang/Driver/Options.td)). Isn't it sufficient?

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