[llvm] [Docs] Document persistent metadata printing (PR #220967)

[Hans Wennborg via llvm-commits]

zmodem wrote:

Interesting!

But I'm still confused, probably because I don't have enough context, but the readers of this release note won't have it either. It sounds like there are now "persistent" and "canonical" metadata node numbers -- the "persistent" ones are new, and the "canonical" ones should match what we had before.

But what is the motivation for having two numberings? Should Rust also do the renumbering before printing, or just update its expectations to match the new numbering?

https://github.com/llvm/llvm-project/pull/220967