ref-manual: variables: adding multiple groups in GROUPADD_PARAM

Add missing documentation on how to add multiple groups with a single
GROUPADD_PARAM:${PN}

(From yocto-docs rev: 8d1aecf5aa638aa98676bda9b30a241c350f94a3)

Signed-off-by: Geoff Parker <geoffrey.parker@arthrex.com>
Reviewed-by: Michael Opdenacker <michael.opdenacker@bootlin.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>

documentation/ref-manual/variables.rst

@@ -3234,6 +3234,14 @@ system and gives an overview of their function and contents.
 
          GROUPADD_PARAM:${PN} = "-r netdev"
 
+      More than one group can be added by separating each set of different
+      groups' parameters with a semicolon.
+
+      Here is an example adding multiple groups from the ``useradd-example.bb``
+      file in the ``meta-skeleton`` layer::
+
+         GROUPADD_PARAM:${PN} = "-g 880 group1; -g 890 group2"
+
       For information on the standard Linux shell command
       ``groupadd``, see https://linux.die.net/man/8/groupadd.