docs: hidden template variables don't have an obvious default

[chri2547]

They were using a hidden template variable, so were unclear what the hidden variable would default to in their query, and there is no clear way to set a default.

You wanted to use all the values that a hidden Templat Variable query returned.
We identified that the "allow all "option was not selected. Once that was selected, the "all" option was used for the default value for the hidden variable query.

We agreed that this solved your particular problem, and that I would investigate if this is a UI but, Documentation bug, or potential low urgency Feature Request

[chri2547]

Refer to https://github.com/grafana/support-escalations/issues/1546 for details.

[rakshit087]

https://github.com/grafana/support-escalations/issues/1546

the link is broken.