In this case the correct docs are Perl's :D
We're borrowing pack/unpack from them anyways, and their explanations go more in depth. Maybe we should link to them...

https://perldoc.perl.org/functions/pack.html

  a  A string with arbitrary binary data, will be null padded.
  Z  A null-terminated (ASCIZ) string, will be null padded.

described as

  The a , A , and Z types gobble just one value, but pack it as a string of length
  count, padding with nulls or spaces as needed. When unpacking, A strips trailing
  whitespace and nulls, Z strips everything after the first null, and a returns
  data with no stripping at all.

  If the value to pack is too long, the result is truncated. If it's too long and
  an explicit count is provided, Z packs only $count-1 bytes, followed by a null
  byte. Thus Z always packs a trailing null, except when the count is 0.

I think we can fit the terminating NUL and truncation behavior briefly in the table. Something like "NUL-padded string; always includes at least one NUL terminator". (The "new in PHP 5.5" is covered in the Changelog already.) Or just mention the terminator and add a Note?

One more small change to make: the `format` description says
> For a, A, h, H the repeat count specifies how many characters of one data
> argument are taken,
That applies to Z too.