gmirror.8: Remove references rc.early

The manual page of gmirror describes how gmirror providers can be used
for kernel dumps. Unfortunately, the instruction references
/etc/rc.early, which is no longer a part of rc(8).

Remove references to rc.early and suggest creating an rc(8) service
script instead.

Future work: In the Problem Report on Bugzilla, Lawrence Chen suggested
adding example rc(8) scripts to the gmirror. However, those examples
need to be tested before they become official reference examples in the
base. Also, those scripts should probably land directly to /etc/rc.d,
/usr/share/examples/rc.d, or /usr/share/examples/gmirror instead of the
gmirror manual page.

PR:		178818
Reported by:	Lawrence Chen <beastie@tardisi.com>
Fixes:		dd2b024a33 Removal of early.sh
MFC after:	1 week

lib/geom/mirror/gmirror.8

@@ -24,7 +24,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd November 30, 2017
+.Dd July 21, 2022
 .Dt GMIRROR 8
 .Os
 .Sh NAME
@@ -388,21 +388,26 @@ Reading a dump from the mirror on boot will only work if the
 balance algorithm is used (that way
 .Nm
 will read only from the component with the highest priority).
-If you use a different balance algorithm, you should add:
+If you use a different balance algorithm, you should create an
+.Xr rc 8
+script that sets the balance algorithm to
+.Cm prefer ,
+for example with the following command:
 .Bd -literal -offset indent
 gmirror configure -b prefer data
 .Ed
 .Pp
-to the
-.Pa /etc/rc.early
-script and:
+Make sure that
+.Xr rcorder 8
+schedules the new script before
+.Xr savecore 8 .
+The desired balance algorithm can be restored later on
+by placing the following command in
+.Xr rc.local 8 :
 .Bd -literal -offset indent
 gmirror configure -b round-robin data
 .Ed
 .Pp
-to the
-.Pa /etc/rc.local
-script.
 The decision which component to choose for dumping is made when
 .Xr dumpon 8
 is called.