[docs]: Specifying multiple merge types does not appear to work like a stack

# Documentation request

At https://cloudinit.readthedocs.io/en/latest/reference/merging.html#specifying-multiple-types-and-what-this-does, it appears that specifying the merge type header in the first part of a multipart MIME message should take effect for all future parts (works like a stack).

In trying this with `write_files`, it did not work. I had to specify the same merge type header (specifically, `list(append)+dict(no_replace,recurse_list)+str()`) with each part for it to work as expected.

Version: `cloud-init 23.2.2-0ubuntu0~22.04.1`