Instead of spreading fear and doubt (" Private wikis behind a corporate firewall are especially vulnerable. This is not recommended. ") the documentation should provide a more precise description of possible attack vectors.
1) There may, particularly in private wikis with a restricted number of users and particular forms of trust, other scenarios, where this description might not apply at all.
2) There may be easy possibilities to solve the problem otherwise.
However: This needs a more detailed description of what the author if these lines had in mind when writing this "whuhou - this is all dangerous" kind of remark.