Improve import attachments documentation

Summary

The following documentation should be improved outlining the exact steps customers need to perform to import only their attachments to Jira cloud.

Also, it would be good if we could warn customers that they can split the file and perform the import in parts so it won't timeout.

• https://confluence.atlassian.com/cloudkb/how-to-import-only-part-of-a-jira-export-into-jira-cloud-859461966.html