ref-manual/variables.rst: document the MOUNT_GROUP variable

(From yocto-docs rev: 959735468264bac1bdd7b274fc1477635817c208)

Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/ref-manual/variables.rst

@@ -5739,6 +5739,11 @@ system and gives an overview of their function and contents.
       specifies the base directory for auto-mounting filesystems. The
       default value is "/run/media".
 
+   :term:`MOUNT_GROUP`
+      On non-systemd systems (where ``udev-extraconf`` is being used),
+      specifies the mount group for auto-mounting filesystems. The
+      default value is "disk".
+
    :term:`MULTIMACH_TARGET_SYS`
       Uniquely identifies the type of the target system for which packages
       are being built. This variable allows output for different types of