backends: Add comment to translators in "Switch monitor" string

Without context it's not clear to what it actually refers to. Add some
context.

https://bugzilla.gnome.org/show_bug.cgi?id=770929

src/backends/meta-input-settings.c

@@ -1704,6 +1704,9 @@ meta_input_settings_get_pad_button_action_label (MetaInputSettings  *input_setti
         return accel;
       }
     case G_DESKTOP_PAD_BUTTON_ACTION_SWITCH_MONITOR:
+      /* TRANSLATORS: This string refers to an action, cycles drawing tablets'
+       * mapping through the available outputs.
+       */
       return g_strdup (_("Switch monitor"));
     case G_DESKTOP_PAD_BUTTON_ACTION_HELP:
       return g_strdup (_("Show on-screen help"));