tslog.4: Document what TSLOG excels at

Based on an email mhorne@ sent to arch@.

Reviewed by: debdrup, pauamma_gundo.com
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D34565

(cherry picked from commit 9e337d53dee685327961b8a070d477ef97b85b86)