[llvm] [LangRef] Try to clarify some Metadata semantics (PR #81948)

[Jeremy Morse via llvm-commits]

https://github.com/jmorse approved this pull request.

LGTM. It's hard to pin down the details of `ValueAsMetadata` and `MetadataAsValue` because they don't follow principled rules (AFAIUI) and exist to glue the Value and Metadata hierarchies together at various points. It's the portion of debug-info that isn't a first-class element of LLVM, so it's no surprise it's hard to document.

https://github.com/llvm/llvm-project/pull/81948