[docs] Add examples for printing asynchronous stack for coroutines

Previously when I wrote this document, I felt the completed scripts was
lengthy, redundant and not easy to read. So I didn't add complete
examples in the previous commit.

However, in the recent discussion with @avogelsgesang, I found people
may not know how to use debugging scripts to improve their debugging
efficiency. So now, I feel like it is helpful to put the examples even
if they are a little bit long.

Test Plan: make docs-clang-html

Reviewed By: avogelsgesang

Differential Revision: https://reviews.llvm.org/D132451