Make the conservertive stack scanner calculates `Temporary<T>` as root instead of calling root methods.

See: https://github.com/mozilla/servo/issues/2650
auto merge of #2652 : saneyuki/servo/root, r=jdm

fix #2650
Use '//!' rather than '///' for the overview comment in js.rs.
Add some blank lines to ensure rustdoc picks up the lists.
Correct formatting in documentation in js.rs.