tslog.4: Document TSLOG

Based on commit messages and Colin Percival's presentation
from BSDCan 2018 [1].

[1]: https://papers.freebsd.org/2018/bsdcan/percival-Profiling_the_FreeBSD_kernel_boot.files/percival-Profiling_the_FreeBSD_kernel_boot.pdf

Reviewed by: cperciva, debdrup
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D34430