bst build --track documentation is unclear

Summary

According to bst build --help output, the --track option can be specified multiple times to specify which elements to track.

This is not entirely true, the --track option is recursive, and must be used in conjunction with --track-except.

This is actually quite destructive when the user doesn't know in advance (as I had forgotten).

It would be better to have a separate option for tracking single elements from the option for tracking recursively, but at least the help output (and consequently documentation and man pages), needs to be fixed.