Add freeze/thaw description to devctl(8)

This is a follow-up to 5fa2979791 .

PR:		256311
Reviewed by:	imp
MFC after:	3 days
Differential Revision:	https://reviews.freebsd.org/D29867

usr.sbin/devctl/devctl.8

@@ -24,7 +24,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd April 4, 2019
+.Dd June 1, 2021
 .Dt DEVCTL 8
 .Os
 .Sh NAME
@@ -67,6 +67,10 @@
 .Op Fl f
 .Ar device
 .Nm
+.Cm freeze
+.Nm
+.Cm thaw
+.Nm
 .Cm reset
 .Op Fl d
 .Ar device
@@ -170,6 +174,23 @@ the device will be deleted even if it is physically present.
 This command should be used with care as a device that is deleted but present
 can no longer be used unless the parent bus device rediscovers the device via
 a rescan request.
+.It Cm freeze
+Freeze probe and attach processing initiated in response to drivers being
+loaded.
+Drivers are placed on a
+.Do
+frozen list
+.Dc
+and processed when a later
+.Do
+thaw
+.Dc
+occurs.
+.It Cm thaw
+Resume (thaw the freeze) probe and attach initiated in response to drivers
+being loaded.
+In addition to resuming, all pending actions that were frozen during the freeze
+are performed.
 .It Xo Cm reset
 .Op Fl d
 .Ar device