There isnt anything in the description about it as far as i know, i only have a vague memory of some patch notes mentioning it.
Aye, it doesn’t say it explicitly in the game, though it does on the wiki - Waystalker - Vermintide 2 Wiki
Yes the talent is unclear, as are many talent descriptions.
But the bleed on Waystalker helps her to hit many useful breakpoints with longbow as well as swiftbow.
I am not a high level player so it may work out differently at endgame, but due to the lack of scaling, I find it underwhelming compared to the other two talent options when using a longbow. When using one I am usually sniping enemies and dropping them in a single shot, so the serrated DoT just gets wasted most of the time.
To get back on topic, I would assume when used with a Hagbane bow, it would apply whichever ends up doing more damage, which in this case would be the Poison.