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GH-36621: [C++] Add documentation for ACERO_ALIGNMENT_HANDLING (#36622)

### Rationale for this change

To document an existing environment variable that was not documented.

### What changes are included in this PR?

Adds a documentation section for this environment variable.

### Are these changes tested?

No, there are only docs.

### Are there any user-facing changes?

New documentation
* Closes: #36621

Lead-authored-by: Weston Pace <weston.pace@gmail.com>
Co-authored-by: Antoine Pitrou <pitrou@free.fr>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>