Make descriptions in Jail chapter more concise, add references

These changes are based on suggestions by vale:

• add conciseness
• avoid the use of won't and use "will not" instead (and others like it)
• Do not start sentences with a reference

Originally, this was part of a patch to add updated FreeBSD versions to
the jails chapter. I've split those out and committed them in
e4fc6b8657422d02cb1eddb166b928cf851be5ac separately.

Differential Revision: https://reviews.freebsd.org/D49038