The more times I read the documentation and your comments I can see that the implementation is OK and the documentation is "complete" and can be read correctly. Please take this as constructive feedback to improving the documentation to make it easier to understand and harder to read incorrectly.

Yes, adding a "Standard size" column would have been very helpful. (I had missed the section on "standard" sizes.)

"Standard" is a very general term. And slightly confusing that standard isn't the default. Could the term "platform independent" (or "fixed"?) be added as an explanation of "standard" - or perhaps used instead?

Programming skills and platform knowledge at C level should not be a requirement to understand and use struct, so perhaps the references to C should be less high-profile, and perhaps something like this could be added:
"All sizes except trivial 1-byte entries (whatever that means) are platform dependent - use calcsize to get the size on your platform."

Perhaps the sections explaining 's', 'p', 'ILqQ', 'P' and '?' could be changed to (foot)notes to the table to make it easier to see where they belongs and if they can be skipped.

Perhaps "@" in the byte order table could be replaced with "@ (default)"? (And perhaps drop "If the first character is not one of these, '@' is assumed.")

The byte order character must come first in the format string and is a key to understand the other format characters, so perhaps everything related to that should come first?