# Which issue does this PR close?

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- Closes apache#9287

# Rationale for this change

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As identified by the issue, it is possible to use safe APIs to produce
an unaligned buffer in `BufferBuilder`.

# What changes are included in this PR?

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Mark `BufferBuilder::new_from_buffer` as unsafe since there are
alignment invariants that must be upheld to use it correctly.

Also fix some docstrings.

# Are these changes tested?

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Marking API unsafe only.

# Are there any user-facing changes?

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Yes.