DOCS - Repeating panels require variable to have one or more items (#23080)

* add note that it's not supported to repeat a panel zero times
see https://github.com/grafana/grafana/issues/23036#issuecomment-603479142

Signed-off-by: David Ellis <ellisda@gmail.com>

* Update docs/sources/reference/templating.md

Co-authored-by: Diana Payton <52059945+oddlittlebird@users.noreply.github.com>

docs/sources/reference/templating.md

@@ -407,6 +407,8 @@ By choosing `vertical` the panels will be arranged from top to bottom in a colum
 Only make changes to the first panel (the original template). To have the changes take effect on all panels you need to trigger a dynamic dashboard re-build.
 You can do this by either changing the variable value (that is the basis for the repeat) or reload the dashboard.
 
+> **Note:** Repeating panels require variables to have one or more items selected; you cannot repeat a panel zero times to hide it.
+
 ## Repeating Rows
 
 As seen above with the *Panels* you can also repeat *Rows* if you have variables set with  `Multi-value` or