-B should not be used with an old nfsroot

man/fai-mirror.1

@@ -1,7 +1,7 @@
 .\"                                      Hey, EMACS: -*- nroff -*-
 .if \n(zZ=1 .ig zZ
 .if \n(zY=1 .ig zY
-.TH fai-mirror 1 "November 2015" "FAI 5.0"
+.TH fai-mirror 1 "October 2023" "FAI 6.0"
 .\" Please adjust this date whenever revising the manpage.
 .\"
 .\" Some roff macros, for reference:
@@ -59,7 +59,9 @@ architecture of host.
 Do not add packages included in base.tar.xz and packages defined in
 /etc/fai/NFSROOT to the mirror. By default all these packages are
 included in the mirror. This will explicitly remove packages from the
-mirror that are already in base.tar.xz.
+mirror that are already in base.tar.xz. You should not this flag if your
+nfsroot is older and newer versions of the packages included in
+base.tar.xz should be included into the mirror.
 .TP
 .BI \-b
 Create the mirror only from packages names in /srv/fai/config. Do not