init.8: document init_path among loader tunables

Reviewed by:	imp, jilles
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D56536

sbin/init/init.8

@@ -396,6 +396,11 @@ back to the default, so that the
 .Pa /etc/rc
 script is executed with the standard shell
 .Pa /bin/sh .
+.It Va init_path
+Specifies the path used to search for the init binary.
+More than one binary can be listed in the string,
+candidate file names are split by the \[lq]:\[rq] separator.
+Each name is opened in sequence until a valid executable is found.
 .El
 .Sh FILES
 .Bl -tag -width /var/log/init.log -compact