jexec man: Explain how PATH is adjusted.

Add a sentence that explains that PATH is set to /bin:/usr/bin.
I had to dive into jexec.c to find this out.

Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D35251

(cherry picked from commit 7eb6a92e3f938d419d3edc0399fc4c1d1018da15)