Update docs: describe possible terminators for ScopeOp

[bcardosolopes]

          Got it. The [doc](https://llvm.github.io/clangir/Dialect/ops.html#cirscope-cirscopeop) seems to be a little outdated.

Originally posted by @Kuree in https://github.com/llvm/clangir/pull/262#discussion_r1329448293