Document the existence of machdep.guessed_bootdev, as suggested

by Sheldon.
For a detailed description look at the commit log for sysctl.c
rev.1.42 -- i do not think it is appropriate to put the full
description in this manpage, and the "boot" and "loader" manpages
where this description might go are also missing a description of
a number of similar variables, so i think this it is ok to limit
documentation to this now, and update it later when I (or someone
else) have a chance to revise "boot" and "loader".

sbin/sysctl/sysctl.8

@@ -196,6 +196,7 @@ denote
 .It "machdep.console_device	dev_t	no
 .It "machdep.adjkerntz	integer	yes
 .It "machdep.disable_rtc_set	integer	yes
+.It "machdep.guessed_bootdev	string	no
 .It "user.cs_path	string	no
 .It "user.bc_base_max	integer	no
 .It "user.bc_dim_max	integer	no