My wiki users are confused about adding TemplateData. We use the Wikipedia {{documentation}} template. My users often add <templatedata> to both the template itself and the /doc subpage, without meaning to, because the "Manage TemplateData" button appears on both. I know you display a warning about it on the template page, but it would be better if the button appeared on only one of the pages. So here is a suggested solution.
Please allow a wiki to officially designate a particular subpage for template documentation. For example:
//LocalSettings.php: $wgTemplateDataSubpage = "doc";
If this variable is set, then the "Manage TemplateData" button would appear only when editing the /doc subpage, not when editing the template itself. This prevents users from writing redundant <templatedata> blocks on the template + the subpage.
I suspect you can dream up other interesting applications, if the wiki has a known standard subpage for docs.