Document keybinding for activating multiple entries

wiki/src/doc/first_steps/accessibility.mdwn

@@ -43,7 +43,8 @@ To access the universal access menu using the keyboard:
    the menu.
 
 1. Press <span class="button">Enter</span> to toggle the selected
-   item.
+   item. To select multiple items without closing the menu,
+   activate or deactivate the selected item with <span class="button">Space</span>.
 
 Screen reader
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