Enhance documentation of `zip_source_zip_file` and `zip_source_zip` about libzip HOT 2 OPEN

To answer your questions:

1. Yes, that's no problem.
2. No, passing NULL would crash. If you pass NULL as error to zip_source_zip_file_create(), the error information is lost.
3. There are more zip_source_foo() / zip_source_foo_create() function pairs that all take archive or error as arguments, and keeping them consistent seems like a good idea. We'll add a paragraph to all their man pages to document these arguments.

If you pass NULL as password, the default password for the archive is used. If you pass the empty string, that is used as password.

from libzip.

If you pass NULL as password, the default password for the archive is used. If you pass the empty string, that is used as password.

This might be worth documenting very clearly. In particular, this is a place where a NULL string and an empty string have different meanings and effects. This can be quite tricky.

This is also subtly different from the way zip_set_default_password treats its password parameter. See zip_set_default_password's documentation:

If password is NULL or the empty string, the default password is unset.

In that case, a NULL or an empty string have the same meaning and effect.

from libzip.