"Focusable element has no keyboard trap" [80af7b]: fix Failed example and add clarification

[kengdoj]

Updates were made to an atomic rule in #1905 for "Focusable element has no keyboard trap via standard navigation" [a1b64e]

• Fix Failed Example 1 to ensure there actually is a keyboard trap
• Add a clarification about forward and backward navigation

Making the same updates for the composite rule.