auto merge of #12298 : alexcrichton/rust/rustdoc-testing, r=sfackler

It's too easy to forget the `rust` tag to test something.

Closes #11698

diff --cc src/doc/rustdoc.md
Simple merge

diff --cc src/libextra/json.rs
Simple merge

diff --cc src/librustdoc/test.rs
Simple merge

diff --cc src/libstd/fmt/mod.rs
Simple merge

diff --cc src/libstd/task.rs
Simple merge

diff --cc src/libstd/vec.rs
Simple merge