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

[Ryotaro Kasuga via cfe-commits]

kasuga-fj wrote:

As far as I can tell, it doesn't seem like a link to UsersManual is there, but you can reach both from [the top page](https://clang.llvm.org/docs/index.html).

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