`pad_XXX`/`align_XX` attributes write unexpected zeros.

[ismell]

Take the following:

pub struct BootImage {
    pub header: BootImageHeader,

    // Skip reading the individual ramdisk fragments.
    #[brw(pad_size_to = header.ramdisk_size, align_after = header.page_size)]
    pub ramdisk_fragments: (),

   ...
}

I'm reading BootImage from a file, changing a couple properties, and writing BootImage back to the same file. I was surprised to find out that the ramdisk_fragments part of my file was all zeros. I wanted to skip reading/writing the ramdisk_fragments since it's not relevant to my use case.

Reading the documentation for align_before, align_after, and pad_size_to, it says "Aligns the reader/writer to the Nth byte before a field." and "Ensures the reader/writer is always advanced at least N bytes.". It doesn't mention that it zero fills the space. Reading the pad_after function shows the zeroing code.

I doubt we can change the behavior at this point, but maybe we can update the documentation?

I can work around my issue by using seek_before, it just makes the struct less readable.

Thanks!