This question is inspired by the recent public reveal of the Stack Overflow Question Close Reasons - Definitions and Guidance. Briefly, the Q&A provides really useful community-specific guidance on what close vote reasons should be used for what problem.
Can this be added to the flag posts privilege wiki?
The help pages for each privilege are identical network-wide, so I don't think they can be customized for a specific site. (Even if it were possible, creating a site-specific override of a help center page's text would mean that the site-specific version couldn't be automatically updated anytime the text changed in the network-wide version of that help page.)
That's fair, but it wouldn't be as big of a problem if community moderators were able to edit each site's privilege pages.
In this answer Tim Post noted:
[Making additional pages in the help center mod-editable] ... is unlikely to happen, but not impossible
I think that
help/closed-questions are particularly ripe for community-specific guidance.