docs: driver-model: device: Add ATTRIBUTE_GROUPS() example

Add a paragraph about the ATTRIBUTE_GROUPS() helper macro, and use it in
the example.  Retain the old description, as it is still useful in case
of multiple groups.  Change the names of the group(s) structures, to
match the ATTRIBUTE_GROUPS() macro.
Signed-off-by: NGeert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/20210303133845.3939403-3-geert+renesas@glider.beSigned-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/driver-api/driver-model/device.rst

@@ -82,19 +82,24 @@ organized as follows into a group::
 	NULL,
   };
 
-  static struct attribute_group dev_attr_group = {
+  static struct attribute_group dev_group = {
 	.attrs = dev_attrs,
   };
 
-  static const struct attribute_group *dev_attr_groups[] = {
-	&dev_attr_group,
+  static const struct attribute_group *dev_groups[] = {
+	&dev_group,
 	NULL,
   };
 
+A helper macro is available for the common case of a single group, so the
+above two structures can be declared using:::
+
+  ATTRIBUTE_GROUPS(dev);
+
 This array of groups can then be associated with a device by setting the
 group pointer in struct device before device_register() is invoked::
 
-        dev->groups = dev_attr_groups;
+        dev->groups = dev_groups;
         device_register(dev);
 
 The device_register() function will use the 'groups' pointer to create the