https://debbugs.gnu.org/cgi/bugreport.cgi?bug=19002 shows a place where a
consumer of gnupg's machine-parseable forms expected that gpg would always
produce the exact same number of fields.

This is clearly a bug with their code, but it would be useful to describe what
sorts of changes users of machine-consumable data should be able to cope with.

For example:

• The currently defined fields are: […] Note that in the future, new fields may be added to the end of the line.
• some fields of some data types are currently unpopulated or have no specific

definition. Future versions may define and/or populate these fields.

Implementers consuming this output should be prepared to ignore fields that they
don't know about.