docs: Improve HowToSetUpLLVMStyleRTTI.

* Fix confusing explanation regarding abstract classes.

* Clarify auto-upcasting and why `Shape` doesn't need a `classof()`.

* Add section `Rules of Thumb` with some quick summary tips.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@165768 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed