find(1): Refine the HISTORY within the manual page.

A simple find command appeared in Version 1 AT&T UNIX and was removed in
Version 3 AT&T UNIX. It was rewritten for Version 5 AT&T UNIX and later
be enhanced for the Programmer's Workbench (PWB). These changes were
later incorporated in AT&T UNIX v7.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D29114

(cherry picked from commit a9275d996c229a30879baa42a6d02d24663ac43b)