[rustdoc] Don't document stripped items in JSON renderer.
Fixes #80664, see [my comment there](https://github.com/rust-lang/rust/issues/80664#issuecomment-797557948) for why
Note that we already do something similar in `convert_item`:
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|---|---|---|
| .. | ||
| cache.rs | ||
| context.rs | ||
| mod.rs | ||
| print_item.rs | ||
| tests.rs | ||
| write_shared.rs | ||