doc: general improvements to tls.md copy

Restructuring and clarifications to the tls.md copy
to improve readability and flow.

PR-URL: #6933
Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Shigeki Ohtsu <ohtsu@iij.ad.jp>