intro.5: More verbose introductory sentence

Explain a little about everything is a file,
matching the style of earlier intro pages.

PR: 248562
MFC after: 3 days
Approved by: mhorne (mentor)
Reviewed by: imp, mhorne, Lexi Winter <lexi@le-fay.org>
Pull Request: https://github.com/freebsd/freebsd-src/pull/1534