Expand why a custom backup filename helps with rsync and incremental backup

The following discussion from !123992 (merged) should be addressed:

• @sranasinghe started a discussion: (+1 comment)

  The resulting file is named `dump_gitlab_backup.tar`. This is useful for
  systems that make use of rsync and incremental backups, and results in
  considerably faster transfer speeds.

  @mkozono This statement probably doesn't provide enough context in relation to rsync and incremental backups and how it speeds things up. Can we look at providing more details of why and how? We should also provide an link to the incremental backup section.