ref-manual/kickstart.rst: add extra-partition-space

After commits 47533f20a7ec ("wic: rename wks flag --extra-space to
--extra-filesystem-space") and c7f51c23fd268 ("wic: add
--extra-partition-space option to set unused space") in openembedded-core,
document the new Wic-specific option extra-partition-space and rename
the extra-space option to extra-filesystem-space.

CC: Alexander Kanavin <alex.kanavin@gmail.com>
(From yocto-docs rev: cba5dbb32a24c9e20e5a6066ec382cf7ff9b93e2)

Signed-off-by: Pierre-Loup GOSSE <pierre-loup.gosse@smile.fr>
Reviewed-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/ref-manual/kickstart.rst

@@ -158,7 +158,7 @@ the ``part`` and ``partition`` commands:
    excludes the given relative path from the resulting image. This
    option is only effective with the rootfs source plugin.
 
--  ``--extra-space``: This option is a Wic-specific option that adds
+-  ``--extra-filesystem-space``: This option is a Wic-specific option that adds
    extra space after the space filled by the content of the partition.
    The final size can exceed the size specified by the ``--size``
    option. The default value is 10M. Specify as an integer value
@@ -166,6 +166,16 @@ the ``part`` and ``partition`` commands:
    for mebibyte and "G" for gibibyte. The default unit if none is given
    is "M".
 
+-  ``--extra-partition-space``: This option is a Wic-specific option that adds
+   extra empty space after the space filled by the filesystem.
+   With ``--fixed-size`` option, the extra partition space is removed from the
+   filesystem size. Otherwise (with or without ``--size`` flag), the extra
+   partition space is added to the final partition size.
+   The default value is 0M. Specify as an integer value
+   optionally followed by one of the units "k" / "K" for kibibyte, "M"
+   for mebibyte and "G" for gibibyte. The default unit if none is given
+   is "M".
+
 -  ``--overhead-factor``: This option is a Wic-specific option that
    multiplies the size of the partition by the option's value. You must
    supply a value greater than or equal to "1". The default value is