toolkitpromise-992309: Merge changes from mozilla-central

Protect promise in menu.js from fast calling of destroy(). Remove text in
help.js which wasn't l10n-able. Add check in api.load() to avoid churn when
no action was needed.

Signed-off-by: Joe Walker <jwalker@mozilla.com>
refactor-864370: Fix documentation in helpers.js

Signed-off-by: Joe Walker <jwalker@mozilla.com>