fix ^ evaluates as bitwise XOR instead of exponentiation - #22314

## Which issue does this PR close?

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- None; documentation fix. Follows up apache#22314.

## Rationale for this change

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The operators page documents `#` as bitwise xor, "interchangeable with
`^`". On `main`, `SELECT 5 # 3;` fails with the default dialect
(`ParserError("No infix parser for token Sharp ...")`); `#` is only xor
in the PostgreSQL and Redshift dialects. The page describes the default
dialect, where `^` is xor.

## What changes are included in this PR?

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The xor section and its index entry now document `^` instead of `#`,
with the current column name (`BIT_XOR`) in the example output. Per
review, the page sticks to the default dialect rather than listing
per-dialect exceptions.

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Ran the new example against `datafusion-cli` built from `main`;
`./ci/scripts/doc_prettier_check.sh` passes. Docs only, no test.

## Are there any user-facing changes?

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Documentation only.

Generated with Claude Code (claude-opus-5-5). It came from running the
documented SQL examples against `main`, not from an issue.