libzfs_sendrecv: Style pass on dump_filesystems

• Add a high level comment.
• Eliminate unnecessarily void arg.
• Capitalize and punctuate complete sentences in comments.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <freqlabs@FreeBSD.org>
Closes #12967