docs: clarify mixed comparison/equality operators

[Elena343-ai]

Summary

Document that mixed consecutive comparison/equality operators such as a < b == c and a == b < c are also unsupported on the comparison operators page, and show the supported conjunction/parenthesized alternatives.

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Type of change

• Bug fix (fixes incorrect behavior)
• New feature
• Performance improvement
• Documentation change with concrete technical impact
• Style, wording, formatting, or typo-only change

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Why is this change needed?

The comparison operators page currently explains that a < b < c is unsupported, but it does not mention that mixed consecutive comparison/equality operators are rejected as well. Because the same page also says that comparison and equality operators share a precedence level, readers can reasonably infer that forms like a < b == c are valid. The parser rejects these expressions with E1028.

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What was the behavior or documentation before?

The page only documented unsupported chained comparisons such as a < b < c.

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What is the behavior or documentation after?

The page also documents unsupported mixed forms such as a < b == c and a == b < c, and
shows explicit conjunction/parenthesized alternatives.

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Related issue or discussion (if any)

N/A

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Additional context

[reviewable-StarkWare]

This change is 

[orizi]

@orizi reviewed 1 file and all commit messages, and made 1 comment.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on Elena343-ai).