add info about third paramter in FAI_DEBOOTSTRAP

man/nfsroot.conf.5

@@ -1,5 +1,5 @@
 .\"                                      Hey, EMACS: -*- nroff -*-
-.TH nfsroot.conf 5 "January 2014" "FAI 5.0"
+.TH nfsroot.conf 5 "September 2022" "FAI 6.0"
 .\" Please adjust this date whenever revising the manpage.
 
 .SH NAME
@@ -17,9 +17,10 @@ used inside the nfsroot. Use bash(1) syntax for defining variables.
 .SH VARIABLES
 .TP
 .B FAI_DEBOOTSTRAP
-Two parameters for debootstrap(8), space separated. Debootstrap is
+Two or three parameters for debootstrap(8), space separated. Debootstrap is
 called as the first part of fai-make-nfsroot unless -B is used. This variable defines
-SUITE and MIRROR for debootstrap. Example:
+SUITE and MIRROR for debootstrap. The third parameter is
+the name of the bootstrap script variant to use. Example:
 
 FAI_DEBOOTSTRAP="stretch http://deb.debian.org/debian"