Add note for printing all dbgmsg entries on FreeBSD

I looked for a bit, and couldn't find any documentation on
how to print all logged dbgmsg entries, just messages since
the DTrace probe started, until @allanjude kindly pointed me
toward the sysctl.

So let's add that note where the DTrace probe is mentioned for
FreeBSD, so other people can find it.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Allan Jude <allan@klarasystems.com>
Signed-off-by: Rich Ercolani <rincebrain@gmail.com>
Closes #12113