doc: add **Note**: to make quote block like a note

Author: weihanglo

src/doc/src/reference/cargo-targets.md

@@ -100,7 +100,8 @@ parallel, reporting the success and failure of each test. See [the `harness`
 field](#the-harness-field) if you want to use a different harness or test
 strategy.
 
-> There is another style of test in Cargo: [documentation tests][documentation examples].
+> **Note**: There is another special style of test in Cargo:
+> [documentation tests][documentation examples].
 > They are handled by `rustdoc` and have a slightly different execution model.
 > For more information, please see [`cargo test`][cargo-test-documentation-tests].