Remove Misleading BooleanBufferBuilder::reserve Doc

[tv42]

Describe the bug

https://docs.rs/arrow/latest/arrow/array/struct.BooleanBufferBuilder.html#method.reserve says

pub fn reserve(&mut self, additional: usize)
Reserve space to at least additional new bits. Capacity will be >= self.len() + additional. New bytes are uninitialized and reading them is undefined behavior.

Other reserve methods look similar, but don't document their undefined behavior; I haven't carefully read the code on whether they actually are unsafe or not.

It seems this is letting the caller trigger undefined behavior, but is not marked as unsafe.

Their documentation should probably also be unified.

• BooleanBufferBuilder::reserve
• OffsetBufferBuilder::reserve
• BufferBuilder.html::reserve
• MutableBuffer::reserve

[tustvold]

It seems this is letting the caller trigger undefined behavior, but is not marked as unsafe.

I think the docs are potentially misleading, it is undefined behaviour to read the allocated bytes, but the only way to so is using unsafe code.

These methods function similarly to Vec::Reserve in that the safe APIs only expose access to the slice that has been initialized.

I think we should remove the New bytes are uninitialized and reading them is undefined behavior. as it is just misleading.

[tv42]

Ah yes, as_slice only exposes up to self.len etc.

[WaterWhisperer]

take