Explain pruning of empty directories

[kurowski]

Re: Backups

Priority: Low

Issue Description:

Pantheon file backups do not include empty directories. This is counter-intuitive, since the backup filename ends in .tar.gz, and the standard behavior of tar is to include empty directories.

Suggested Resolution

The documentation currently explains that "Various content generated by your application is not backed up" such as generated images and aggregated JS/CSS.

It would be useful to explain that empty directories are not backed up. It may be useful to explain in more detail exactly which "various content" is not backed up.

[github-actions]

👋 @kurowski
Thanks for opening your first issue here! @pantheon-systems/docs-admins is excited to review this!