rustdoc: Turn `JsonRenderer::mod_item_in` into `unreachable!()`
rustdoc: Turn `JsonRenderer::mod_item_in` into `unreachable!()`

The JSON renderer no longer gets called on modules.

aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust