docs for incompleteStruct in manual should be improved.
see <https://github.com/nim-lang/Nim/pull/14228>
> the meaning of {.incompleteStruct.} is for types like DIR (eg
> <http://manpages.ubuntu.com/manpages/bionic/man7/dirent.h.7posix.html>) which
> are forward declared, and hence for which sizeof (or accessing members or
> creating an instance on the stack etc) would result in cgen CT error on some
> platforms. Treating it like we used to (via void*) is clearly wrong though.
> Investigating how to best handle this...
there's also `{.completeStruct.}`, but it's unrelated to `{.incompleteStruct.}`
(completeStruct is described in manual, hopefully in more details than
incompleteStruct)
