DescriptionList - heading level and heading element invalid in examples

Describe the problem
This ranges from example code update to component update:

• For the Popover that triggers when clicking the term help text, the heading level is not correct for where it is rendered in the DOM. Either the heading should be an h1 element, or the Popover needs to be appended in a way that the current h6 is correct.

• For the Popover that triggers when clicking the term help text, axe complains about multiple banner landmarks. This is due to the Popover being appended to the document body and outside the main element; whenever a header is rendered outside of it, it becomes a banner landmark. Some ways to fix this could be:

  • update the element to a plain div
  • change where the Popover is appended (could also help resolve the first issue above)
  • since the Popover traps focus and has attributes similar to a Modal, all other content on the page gets set to aria-hidden="true", similar behavior to the Modal component. This might be an issue since we don't render a Backdrop for Popover like we do for Modal, though, so it may not be totally clear that content outside the Popover cannot be interacted with. Also, unlike Modal, you can click outside the Popover to close it, which may not make the aria-modal="true" attribute totally correct.

How do you reproduce the problem?
Provide steps to reproduce. A codesandbox demonstrating the problem is appreciated.

Expected behavior
A clear and concise description of the expected behavior.

Is this issue blocking you?
List the workaround if there is one.

Screenshots
If applicable, add screenshots to help explain the issue.

What is your environment?

• OS: [e.g. iOS]
• Browser [e.g. chrome, safari]
• Version [e.g. 22]

What is your product and what release date are you targeting?

Any other information?

Jira Issue: PF-1132

从链接的 CodeSandbox 开始，检查 DescriptionList 示例以及根据 term 帮助文本渲染的 Popover。检查生成的 DOM 和 axe 检测结果，然后在保留 Popover 已记录行为的同时，解决标题级别和多个 banner landmark 的问题；更新受影响的示例，并验证可访问性检查通过。