Note that biosboot without -A clears the PMBR "active" flag.

While here, fix some grammar and make the selection options
for biosboot a little clearer.

Ride nia@'s Dd bump.

sbin/gpt/gpt.8

@@ -1,4 +1,4 @@
-.\" $NetBSD: gpt.8,v 1.75 2022/04/07 09:06:01 nia Exp $
+.\" $NetBSD: gpt.8,v 1.76 2022/04/07 13:57:44 kre Exp $
 .\"
 .\" Copyright (c) 2002 Marcel Moolenaar
 .\" All rights reserved.
@@ -263,28 +263,43 @@ primary bootstrap program, used during
 The
 .Fl A
 options sets the PMBR partition active.
+This should not normally be necessary,
+but some firmware might require it.
+If
+.Fl A
+is omitted, the active flag will be cleared from the PMBR label.
 .Pp
 The
 .Fl c
-option allows the user to specify the filename that
+option allows the user to specify the filename from which
 .Nm
-should read the bootcode from.
+should read the bootcode.
 The default is to read from
 .Pa /usr/mdec/gptmbr.bin .
 .Pp
+The partition that should contain the primary bootstrap code,
+.Pq similar to that installed via Xr installboot 8
+is selected using the
+.Fl i ,
+.Fl L
+and
+.Fl b
+options.
+One of these three options is required.
 The
 .Fl i
-option selects the partition that should contain the primary
-bootstrap code, as installed via
-.Xr installboot 8 .
+option selects the partition given by the
+.Ar index .
 The
 .Fl L
-option selects the partition by label.
-If there are multiple partitions with the same label, the
-first one found will be used.
+option selects the partition by
+.Ar label .
+If there are multiple partitions with the same label,
+the first one found will be used.
 The
 .Fl b
-options selects the partition by start block.
+option selects the partition starting at block
+.Ar startsec .
 .\" ==== create ====
 .It Nm Ic create Oo Fl AfP Oc Oo Fl p Ar partitions Oc
 The