[Format] Clarify that 8 byte padding must not be applied to compressed buffers

[asfimport]

I was unable to find where this is discussed, but I think we do not mention that 8 byte padding must not be applied when the buffer is compressed, as it causes us to lose the size of the compressed buffer.

For example

import pyarrow.ipc

data = [
    pyarrow.array([1, 2, 3, 4, 5], type="int32"),
]

batch = pyarrow.record_batch(data, names=['f0'])

with pyarrow.OSFile('test1.arrow', 'wb') as sink:
    with pyarrow.ipc.new_file(sink, batch.schema, options=pyarrow.ipc.IpcWriteOptions(compression="zstd")) as writer:
        writer.write(batch)

outputs a single data buffer with

[20, 0, 0, 0, 0, 0, 0, 0, 40, 181, 47, 253, 32, 20, 161, 0, 0, 1, 0, 0, 0, 2, 0, 0, 0, 3, 0, 0, 0, 4, 0, 0, 0, 5, 0, 0, 0]

which has 37 bytes (padding would require 40 bytes).

My understanding is that we do not pad because doing so make us unable to recover the original size of the (compressed) data, and offers no advantage since users can't mmap data anyways.

Reporter: Jorge Leitão / @jorgecarleitao

_{Note: This issue was originally created as ARROW-15687. Please see the migration documentation for further details.}