docs: fix misleading reserve documentation (#9076)

# Which issue does this PR close?

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- Closes #7236 

# Rationale for this change

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The previous documentation for `BooleanBufferBuilder::reserve` mentioned
that reading new bytes is undefined behavior. However, as noted in the
issue, the safe APIs only expose initialized slices, making this warning
misleading.

# What changes are included in this PR?

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- Remove misleading UB warning from `BooleanBufferBuilder::reserve`.
- Unify documentation for the `reserve` methods to follow a consistent
format.

# Are these changes tested?

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No, because it's doc change

# Are there any user-facing changes?

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Yes

Author: WaterWhisperer

arrow-buffer/src/builder/boolean.rs

@@ -140,7 +140,6 @@ impl BooleanBufferBuilder {
 
     /// Reserve space to at least `additional` new bits.
     /// Capacity will be `>= self.len() + additional`.
-    /// New bytes are uninitialized and reading them is undefined behavior.
     #[inline]
     pub fn reserve(&mut self, additional: usize) {
         let capacity = self.len + additional;