Rollup merge of #24860 - Manishearth:cargo-doc, r=steveklabnik
Rollup merge of #24860 - Manishearth:cargo-doc, r=steveklabnik

A user in IRC was having trouble because they used `main.rs` when they were trying
to migrate a library. The `[[lib]]` key is not easily found, and the `main.rs`/`lib.rs`
distinction doesn't seem to exist in trpl


r? @steveklabnik

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