Improve user experience for outdated comments (#29050) (#29086)

Improve user experience for outdated comments (#29050) (#29086)

Backport #29050 by wxiaoguang

Try to improve #28949

1. Make `ctx.Data["ShowOutdatedComments"] = true` by default: it brings
consistent user experience, and sometimes the "outdated (source
changed)" comments are still valuable.
2. Show a friendly message if the comment won't show, then the end users
won't fell that "the comment disappears" (it is the special case when
`ShowOutdatedComments = false`)

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>