Handbook: Preface - Upgrade Fourth Edition section

The preface section has been updated to incorporate the latest changes
made by the Handbook Working Group in preparation for the 4th edition
of the Handbook.

Thanks to:

• arrowd@
• bapt@
• bcr@
• dbaio@
• emaste@
• fernape@
• jrm@
• karels@
• manu@
• mckusick@
• philip@
• The FreeBSD Foundation

Special thanks to karels@, whose contributions and influence
will be remembered.

Sponsored by: Siracusa