manuals: add documentation for WKS_FILES

Fixes [YOCTO #14590]

(From yocto-docs rev: e736ff92b5b09f2028f3e8a5152973941d25dae4)

Signed-off-by: Michael Opdenacker <michael.opdenacker@bootlin.com>
Reviewed-by: Quentin Schulz <foss@0leil.net>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/ref-manual/variables.rst

@@ -8767,6 +8767,19 @@ system and gives an overview of their function and contents.
       previous example, some-native-tool would be replaced with an actual
       native tool on which the build would depend.
 
+   :term:`WKS_FILES`
+      Specifies a list of candidate Wic kickstart files to be used by the
+      OpenEmbedded build system to create a partitioned image. Only the
+      first one that is found, from left to right, will be used.
+
+      This is only useful when there are multiple ``.wks`` files that can be
+      used to produce an image. A typical case is when multiple layers are
+      used for different hardware platforms, each supplying a different
+      ``.wks`` file. In this case, you specify all possible ones through
+      :term:`WKS_FILES`.
+
+      If only one ``.wks`` file is used, set :term:`WKS_FILE` instead.
+
    :term:`WORKDIR`
       The pathname of the work directory in which the OpenEmbedded build
       system builds a recipe. This directory is located within the