Hi Gerard, The best way of letting us know about documentation issues/updates/improvements is by using the Feedback button or by adding a comments to the page in question. When you submit feedback we receive it on our end and forward it to the right person, so they can update the documentation.
To add to Rom's answer: we very much appreciate your feedback and improvements on the documentation, so keep it coming! (with or without replacement text)
I would contact my PSM (partner success manager) and discuss how to deal with these kinds of issues. Perhaps Mendix doesn't want to solve these issues through the support portal and he can direct you to the proper person to submit these issues. Alternatively, he could instruct the support desk to accept these issues.
If that yields no results, I would post the issues you find on the forums (perhaps create a single topic to collect documentation errors): there are quite a few Mendix employees who visit these forums, and I think they will update the documentation accordingly, especially if you supply a replacement text.