[LLVM][Docs] Update for HowToSetUpLLVMStyleRTTI.rst, NFC.

This patch updates the document with some advanced use cases and examples on how to set up and use LLVM-style RTTI. It includes a few motivating examples to get readers comfortable with the concepts.

Reviewed By: lattner

Differential Revision: https://reviews.llvm.org/D126943