It's not about adding markup, it's about how to handle the existing markup - i.e. the <!--T:n--> tags.
For most parts the contributor can just ignore them (modifying existing content), but what if he/she moves some text around or deletes some text? I also think we need to clarify what we mean by "NEVER add new translate section tags.", since some may not realize what <!--T:n--> are for and add them for the new text.
The instructions for handling markup are on the page about preparing for translation, where they belong.
I would still argue that the information belongs on this page as well, at least a mention and link to further information.
I would say that this is even more relevant on this page than on preparing for translation page for the reasons above.
By the way (not directly related to the issue above), I couldn't find the <!--T:n--> tags being mentioned in Edit Markup.
There is already, in the Warning box, a link to EditMarkup and the Typographical_Guidelines. The purpose of that warning is that anyone editing text as described should not be touching the tags at all. I will think about whether it is appropriate to write more about the tags, but it will not be on this page. It is entirely inappropriate to the task.
I have changed the warning slightly.
@annew, the problem is a simple cut and paste to a page section can result in <translate> tags appearing in preview and incomprehensible garbled internal errors like "Unbalanced </translate> tag."
Maybe leave the general instruction, but please can those errors link to something that gives you enough info to fix the problem? (I've spent 25 bloody minutes fighting --T5-- warnings and unmatched </translate> after a 2-line edit, and I've worked with SGML/HTML/XML for 26 years.)