ref-manual/variables.rst: expand IMAGE_OVERHEAD_FACTOR glossary entry

There's a (second) overhead factor applied in images generated with Wic,
and this is already documented in the .wks reference. However, the
IMAGE_OVERHEAD_FACTOR entry does not mention it, and by looking at the
partition sizes (e.g. with parted) one may find it confusing that they
don't match with the expected rootfs size (e.g. in a scenario where the
extra space is "0" and IMAGE_OVERHEAD_FACTOR="1.0").

This second overhead is already documented, though:
https://docs.yoctoproject.org/ref-manual/kickstart.html#command-part-or-partition

Mention the '--overhead-factor' option in the glossary entry and add a
reference to the wks documentation.

(From yocto-docs rev: 71a3933c609ce73ff07e5be48d9e7b03f22ef8d7)

Signed-off-by: Joao Marcos Costa <joaomarcos.costa@bootlin.com>
Reviewed-by: Quentin Schulz <quentin.schulz@cherry.de>
Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
(cherry picked from commit b9040e20b015e9b02683ec3014e4ade5eb59d41a)
Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>

documentation/ref-manual/variables.rst

@@ -3535,6 +3535,12 @@ system and gives an overview of their function and contents.
       added to the image by using the :term:`IMAGE_ROOTFS_EXTRA_SPACE`
       variable.
 
+      When using Wic tool, beware that a second overhead factor is also applied.
+      This overhead value is defined by the ``--overhead-factor`` option, which
+      defaults to "1.3" when omitted. See the
+      :ref:`ref-manual/kickstart:command: part or partition` chapter in
+      :doc:`/ref-manual/kickstart` for details.
+
    :term:`IMAGE_PKGTYPE`
       Defines the package type (i.e. DEB, RPM, IPK, or TAR) used by the
       OpenEmbedded build system. The variable is defined appropriately by