[clang] Improve documented sampling profiler steps to best known methods (PR #88438)

[Tim Creech via cfe-commits]

tcreech-intel wrote:

@williamweixiao, @HaohaiWen, this updates the docs to describe best practices given #83972.

It seems `-fdebug-info-for-profiling` can be particularly important. Without it we were discarding nearly half of the samples in some cases.

https://github.com/llvm/llvm-project/pull/88438