How can we give help tool tip in rich text item ?

I think there is no other way. we can use richtextdisplayfield for tooltip. otherwise we can use rich text item as a tooltip reduce text size. 

For individual items? No. You can create multiple richTextDisplayFields for that matter. 

I'm really not clear why this isn't a feature.  You can apply a tooltip that applies across all text of a single richTextDisplayField(), but for whatever reason, they didn't make it more granular (allowing individual tooltips on granular richTextItem entries).  I would have found that feature very handy, but instead I find I hardly use it at all (apparently most of my use cases are ones where I'd really only want to show a tooltip on part of the text in a component).

Usually I get around this by adding an icon that can have a caption, as a proxy tooltip (and proxy "help icon") - this is pretty easy to do and manage.