rustdoc-json: Document why Id can't just be DefId

[aDotInTheVoid]

NIT: This comment could be more useful. The problem it's actually trying to solve is that a single pub use with 1 defid can expand to multiple rustdoc-json items it the name exists in multiple namespaces: https://godbolt.org/z/jEnGxoWx6.

At some point in the future, we should maybe change how we do this on the JSON side, but not in this PR.

Originally posted by @aDotInTheVoid in #130078 (comment)

Rustdoc-json's Id handling is a bit subtle. There should be a nice doc-comment to explain it.

It came up on zulip here