wp.makyen wrote:

You can use templates to translude portions of the TemplateData JSON object. You do have to use something like {{((}}#tag:templatedata|Your templates here}}. I have set up an example at https://en.wikipedia.org/wiki/Template:Ref_supports. The resulting TemplateData can be seen at https://en.wikipedia.org/w/api.php?action=templatedata&titles=Template:Ref_supports/doc&format=jsonfm. The example transcludes separate portions of the template data JSON from 5 subpages onto the Ref supports/doc page which is then transcluded, using {{documentation}}, onto the template page.

wp.makyen wrote:

I failed to remove the wikitext from the {{#tag}} example. It should have been:
{{#tag:templatedata|Your templates here}}.

(In reply to Makyen from comment #2)

I failed to remove the wikitext from the {{#tag}} example. It should have
been:
{{#tag:templatedata|Your templates here}}.

If you're using #tag to do something, that's generally a really strong sign that you shouldn't be doing it at all. :-) In this case, that completely and utterly breaks any chance of using a proper tool to edit the TemplateData, and won't survive the content being transferred to its own JSON-only namespace.

(In reply to James Forrester from comment #3)

...
and won't survive the content being transferred to its own JSON-only
namespace.

aka bug 54140

I'm not quite sure if this is quite whats wanted but there are already some templates which share the same TemplateData documentation

http://en.wikipedia.org/wiki/Template:Yes
http://en.wikipedia.org/wiki/Template:No

and related all share

http://en.wikipedia.org/wiki/Template:Table_cell_templates/doc

which has a template data section. These share the whole block which is worded to be applicable to all of the templates.