Man page also claims that iov is set to NULL, but you did not do that.
Was there a reason for the change except the man page compliance?

Was there a reason for the change except the man page compliance?

Yes, the idea is to be able to reuse iov and iovlen right away after freeing previous ones:

build_iovec(&iov, &iovlen, ...);
build_iovec(&iov, &iovlen, ...);
...

free_iovec(&iov, &iovlen, ...);

build_iovec(&iov, &iovlen, ...);
build_iovec(&iov, &iovlen, ...);
...

Without this change one has to additionally put

iov = NULL;
iovlen = 0;

after free_iovec.