docs: fix case-sensitivity explanation in highest-available-document-mode

[rajucreate]

Pull request checklist

Make sure you:

• Signed the Contributor License Agreement (after creating PR)
• Followed the commit message guidelines

For non-trivial changes, please make sure you also:

• Added/Updated related documentation.
• Added/Updated related tests.

Short description of the change(s)

This PR improves the behavior of the highest-available-document-mode hint by fixing an incorrect case-sensitive match for the X-UA-Compatible header.

What was the issue?

The existing implementation compared the header value using a case-sensitive check, which caused valid values such as:
X-UA-Compatible: IE=edge
to incorrectly trigger warnings.

What does this PR do?

i) Introduces a proper case-insensitive normalization for header values
ii) Ensures all variants like IE=edge, Ie=EdGe, or ie=edge pass correctly
iii)Removes outdated note in documentation about case-sensitive limitations
iv) Updates README with clear explanation of new case-insensitive behavior

This improves correctness and aligns with Microsoft’s specification that the header is case-insensitive.

Issue reference

Fixes: #6028

Additional notes

-> No major logic changes beyond normalization.
-> Existing test suite may not require updates unless tests expect strictly lowercase matches.

[linux-foundation-easycla]

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