ref-manual: document image-specific variant of INCOMPATIBLE_LICENSE

This has been around without being properly documented since 2019 (!!!),
and is nowadays the preferred method for enforcing license restrictions,
especially since meta-gplv2 is officially obsolete.

(From yocto-docs rev: 4dfef81ac6164764c6541e39a9fef81d49227096)

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Reviewed-by: Michael Opdenacker <michael.opdenacker@bootlin.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>

documentation/ref-manual/variables.rst

@@ -3547,9 +3547,18 @@ system and gives an overview of their function and contents.
    :term:`INCOMPATIBLE_LICENSE`
       Specifies a space-separated list of license names (as they would
       appear in :term:`LICENSE`) that should be excluded
-      from the build. Recipes that provide no alternatives to listed
+      from the build (if set globally), or from an image (if set locally
+      in an image recipe).
+
+      When the variable is set globally, recipes that provide no alternatives to listed
       incompatible licenses are not built. Packages that are individually
       licensed with the specified incompatible licenses will be deleted.
+      Most of the time this does not allow a feasible build (because it becomes impossible
+      to satisfy build time dependencies), so the recommended way to
+      implement license restrictions is to set the variable in specific
+      image recipes where the restrictions must apply. That way there
+      are no build time restrictions, but the license check is still
+      performed when the image's filesystem is assembled from packages.
 
       There is some support for wildcards in this variable's value,
       however it is restricted to specific licenses. Currently only