ref-manual: add new RECIPE_UPGRADE_EXTRA_TASKS variable

(From yocto-docs rev: c80bc42afa40fd9eb5acaba24da3ad3b5915a254)

Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/ref-manual/variables.rst

@@ -7076,6 +7076,11 @@ system and gives an overview of their function and contents.
       in the ":ref:`ref-manual/devtool-reference:checking on the upgrade status of a recipe`"
       section.
 
+   :term:`RECIPE_UPGRADE_EXTRA_TASKS`
+      When upgrading a recipe with ``devtool upgrade``, the variable
+      :term:`RECIPE_UPGRADE_EXTRA_TASKS` specifies a space-delimited list of
+      tasks to run after the new sources have been unpacked.
+
    :term:`RECIPE_SYSROOT`
       This variable points to the directory that holds all files populated from
       recipes specified in :term:`DEPENDS`. As the name indicates,