[EuiCallout][COGNITION]: Document consumer responsibility with `header` prop

[1Copenut]

Description

@cchaos and @1Copenut were discussing the callout heading prop defaulting to a P tag in PR #5870 5870. We opted to leave the default as a P tag for now and open an exploratory ticket for improved awareness. We want consuming applications to be aware callouts should have a relevant heading that is properly nested (H4 if under an H3 for example) when it makes sense.

Making sure callouts have proper headings is especially important if the text is styled in such a way as to suggest prominence or greater importance than paragraph text.

There are two items we'd like to look at specifically:

1. Callout guidance in the EuiCallout docs page outlining why a properly nested heading is important for assistive technology. I'll list some relevant links below.
2. Making the heading prop required by consuming apps in the future. This feels like a good defensive coding strategy. It also has potential to be a breaking change.

Guidance

• Understanding Success Criterion 1.3.1: Info and Relationships
• Heading off confusion: When do headings fail WCAG?

[cchaos]

Task 2 will absolutely be a breaking change 😅 .

[1Copenut]

So labeled! I appreciate the size of the task, and that moving this ahead will require coordination and a good plan.

[github-actions]

👋 Hey there. This issue hasn't had any activity for 180 days. We'll automatically close it if that trend continues for another week. If you feel this issue is still valid and needs attention please let us know with a comment.

[github-actions]

👋 Hi there - this issue hasn't had any activity in 6 months. If the EUI team has not explicitly expressed that this is something on our roadmap, it's unlikely that we'll pick this issue up. We would sincerely appreciate a PR/community contribution if this is something that matters to you! If not, and there is no further activity on this issue for another 6 months (i.e. it's stale for over a year), the issue will be auto-closed.