ref-manual: classes: update description of class 'image_types'

Cite usage of IMGCLASSES variable in class 'image' as found in OE-Core's
commit [1].

[1] 451363438d38 ("classes/recipes: Switch to use inherit_defer")

(From yocto-docs rev: 674f376ece98466d3498459d5b5428a8bfbd5112)

Signed-off-by: Ulrich Ölmann <u.oelmann@pengutronix.de>
Reviewed-by: Michael Opdenacker <michael.opdenacker@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/ref-manual/classes.rst

@@ -1184,13 +1184,17 @@ enables the :ref:`ref-classes-image_types` class. The :ref:`ref-classes-image` c
 ``IMGCLASSES`` variable as follows::
 
    IMGCLASSES = "rootfs_${IMAGE_PKGTYPE} image_types ${IMAGE_CLASSES}"
-   IMGCLASSES += "${@['populate_sdk_base', 'populate_sdk_ext']['linux' in d.getVar("SDK_OS")]}"
+   # Only Linux SDKs support populate_sdk_ext, fall back to populate_sdk_base
+   # in the non-Linux SDK_OS case, such as mingw32
+   inherit populate_sdk_base
+   IMGCLASSES += "${@['', 'populate_sdk_ext']['linux' in d.getVar("SDK_OS")]}"
    IMGCLASSES += "${@bb.utils.contains_any('IMAGE_FSTYPES', 'live iso hddimg', 'image-live', '', d)}"
    IMGCLASSES += "${@bb.utils.contains('IMAGE_FSTYPES', 'container', 'image-container', '', d)}"
    IMGCLASSES += "image_types_wic"
    IMGCLASSES += "rootfs-postcommands"
    IMGCLASSES += "image-postinst-intercepts"
-   inherit ${IMGCLASSES}
+   IMGCLASSES += "overlayfs-etc"
+   inherit_defer ${IMGCLASSES}
 
 The :ref:`ref-classes-image_types` class also handles conversion and compression of images.