Auto merge of #126761 - GuillaumeGomez:unsafe_extern_blocks, r=spastorino

rustdoc: Add support for `missing_unsafe_on_extern` feature

Follow-up of rust-lang/rust#124482.

Not sure if the `safe` keyword is supposed to be displayed or not though? For now I didn't add it in the generated doc, only `unsafe` as usual.

cc `@spastorino`
r? `@fmease`

Author: bors