update prometheus template variables doc (#69335)
* edits for clarity
* added 2 new sections on query variable options
* Update docs/sources/datasources/prometheus/template-variables/index.md
Co-authored-by: Brendan O'Handley <brendan.ohandley@grafana.com>
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Co-authored-by: Brendan O'Handley <brendan.ohandley@grafana.com>
(cherry picked from commit 5ac56bcb28)
Co-authored-by: lwandz13 <126723338+lwandz13@users.noreply.github.com>