IANAL, but as far as I know, the only document that matters in the end is the TOS.
If the TOS has language that even could be interpreted to mean that some of the private data (who you talk to, even message content, etc.) can be shared with Facebook, then WhatsApp is legally covered, despite putting out a separate FAQ or document indicating otherwise. Sure, it would look pretty ******, and definitely would not help in the context of a legal defense (could be seen as misleading or something), but in the end, users agreed to the TOS, not to a FAQ with pretty icons and one-liners.
The verbosity and legalese of the TOS for almost every organization have basically enabled what we have today. Sure there's plenty of us who don't like all the data collection, but ultimately, those who use Facebook (or continue to use any service they buy out and change the TOS for, or honestly, pretty much any tech service today!) agreed to the TOS, and thus agreed to all of this stuff (even if unknowingly). I only see this problem getting worse and worse, as more buyouts, more "We updated our privacy policy" prompts pop up, and more "but ma'private company!" arguments heat up the situation.