MFC: devd.conf(5): explain difference between internal and shell variables

devd.conf by default considers many variables as internal, possibly
expanding them to an empty string. Shell variables thus need to be
wrapped into braces.

Reviewed by: imp, Andre Albsmeier
Differential Revision: https://reviews.freebsd.org/D48154

(cherry picked from commit 1fc7922f9f905b62ef9a6c03be498c282e7b382e)