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MINOR: [Docs][C++] Explain "Illegal Instruction" on legacy CPU (#34528)



### Rationale for this change
We saw "Illegal Instruction" bug repots from time to time. It happens on legacy x86_64 CPUs don't support SSE4.2.
User may misunderstand current document and think that setting `ARROW_USER_SIMD_LEVEL=NONE` will disable SSE instruction at runtime and fix the issue. Clearer explaination is necessary.

### What changes are included in this PR?
Update cpp document to explain "Illegal Instruction" issue and how to fix it.

### Are these changes tested?
No.

### Are there any user-facing changes?
No.

Authored-by: Yibo Cai <yibocai@apache.org>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>