Docs: Time regions add info about UTC timezone (#26407)

* Add UTC info to docs

* Revert formaatting

docs/sources/panels/visualizations/graph-panel.md

@@ -160,4 +160,4 @@ Hide series when all values of a series from a metric query are of a specific va
 
 ### Time regions
 
-Time regions allow you to highlight certain time regions of the graph to make it easier to see for example weekends, business hours and/or off work hours.
+Time regions allow you to highlight certain time regions of the graph to make it easier to see for example weekends, business hours and/or off work hours. All configured time regions refer to UTC time.

public/app/plugins/panel/graph/time_regions_form.html

@@ -83,7 +83,7 @@
 
   <div class="gf-form-button-row">
     <button class="btn btn-inverse" ng-click="ctrl.addTimeRegion()">
-      <icon name="'plus'"></icon>&nbsp;Add time region<tip>All configured time regions refers to UTC time</tip>
+      <icon name="'plus'"></icon>&nbsp;Add time region<tip>All configured time regions refer to UTC time</tip>
     </button>
   </div>
 </div>