No bug: [tryselect] Add docs for `mach try again` to toc-tree; r=ahal

Differential Revision: https://phabricator.services.mozilla.com/D9620

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

tools/tryselect/docs/selectors/index.rst

@@ -31,6 +31,7 @@ See selector specific options by running:
   :hidden:
 
   Fuzzy <fuzzy>
+  Again <again>
   Empty <empty>
   Syntax <syntax>
   Release <release>