The tale of two makeoptions.

There are two forms that were both documented in the selections
section, but the form with the condition (and only that form) can be
used only in the descriptions section.  Move it to the appropriate .Ss
and add prominent notice to both.

usr.bin/config/config.5

@@ -1,4 +1,4 @@
-.\" $NetBSD: config.5,v 1.41 2020/07/31 23:30:23 uwe Exp $
+.\" $NetBSD: config.5,v 1.42 2020/07/31 23:42:21 uwe Exp $
 .\"
 .\"  Copyright (c) 2006, 2007 The NetBSD Foundation.
 .\"  All rights reserved.
@@ -666,6 +666,26 @@ indicates when the relevant line should be added to
 and works just like the
 .Ic file
 statement.
+.\"
+.Pp
+.It Ic makeoptions \
+    Ar condition name Ns Ic += Ns Ar value \
+    Ns Op Ic \&, Ar condition name Ns Ic += Ns Ar value \
+    Ns Op Ic \&, Ar ...
+Appends to a definition in the generated
+.Pa Makefile .
+.Pp
+This variant of
+.Ic makeoptions
+belongs to the options description section.
+The
+.Ar condition
+is mandatory and only
+.Ic +=
+can be used.
+Not to be confused with the the confusingly similar variant of
+.Ic makeoptions
+used in the selections section.
 .El
 .\"
 .\"
@@ -868,14 +888,18 @@ is defined with
 the
 .Ar value
 is defined as an option too.
-.\"
-.\" XXX: This variant of makeoptions can only be specified in the descriptions.
-.It Ic makeoptions \
-    Ar condition name Ns Ic += Ns Ar value \
-    Ns Op Ic \&, Ar condition name Ns Ic += Ns Ar value \
-    Ns Op Ic \&, Ar ...
-Appends to a definition in the generated
-.Pa Makefile .
+.Pp
+This variant of
+.Ic makeoptions
+belongs to the options selection section.
+Both
+.Ic =
+and
+.Ic +=
+can be used.
+Not to be confused with the confusingly similar variant of
+.Ic makeoptions
+used in the descriptions section.
 .\"
 .It Ic no makeoptions Ar name Op , Ar name Op , Ar ...
 Removes one or more definitions from the generated