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GH-43758: [C++] Compute: More comment in RowEncoder (#43763)



### Rationale for this change

Some comments for RowEncoder

### What changes are included in this PR?

Some comments for RowEncoder

### Are these changes tested?

Covered by existing

### Are there any user-facing changes?

no

* GitHub Issue: #43758

Lead-authored-by: mwish <maplewish117@gmail.com>
Co-authored-by: mwish <1506118561@qq.com>
Co-authored-by: mwish <anmmscs_maple@qq.com>
Co-authored-by: Antoine Pitrou <pitrou@free.fr>
Co-authored-by: Rossi Sun <zanmato1984@gmail.com>
Signed-off-by: mwish <maplewish117@gmail.com>