Document installing FreeRADIUS from packages

See the background for this report in support#16875.

In essence, the error observed was:

HOOKS_OPEN_ERROR failed to open hook library /usr/lib64/kea/hooks/libdhcp_radius.so: /usr/lib64/kea/hooks/libdhcp_radius.so: undefined symbol: rc_acct_async

DHCP4_PARSER_FAIL failed to create or run parser for configuration element hooks-libraries: hooks libraries failed to validate - library or libraries in error are: /usr/lib64/kea/hooks/libdhcp_radius.so(/etc/kea/kea-dhcp4.conf:10:5)

However, the underlying problem is that incorrect FreeRADIUS version was installed. It was a plain FreeRADIUS, without Francis' patch. This should be better explained in the docs. Something along "if you see this error, please install the patched FreeRADIUS version".