The current documentation states:
Searches all files in the path for certificates encoded in the specified format and adds them to this socket's CA certificate database. path can be explicit, or it can contain wildcards in the format specified by syntax.

However, it directly calls QSslCertificate::fromPath, and it's documentation states:
Searches all files in the path for certificates encoded in the specified format and returns them in a list. must be a file or a pattern matching one or more files, as specified by syntax.

So just giving a path will not work. This can be fixed in documentation, or the addCaCertificates function could be changed to match it's documentation.