Bug 1617511 - doc: Add a textual description on attaching new try jobs r=armenzg

Differential Revision: https://phabricator.services.mozilla.com/D63817

--HG--
extra : moz-landing-system : lando

tools/tryselect/docs/index.rst

@@ -60,6 +60,8 @@ Phabricator.
 
 This run is created for static analysis, linting and other tasks. Attaching new jobs to the run is
 easy and doesn't require more actions from the developer.
+Click on the down-arrow to access the actions menu, select the relevant jobs
+and, click on ``Trigger X new jobs`` (located on the top of the job).
 
 .. image:: img/add-new-jobs.png