[PATCH] ref-manual: explain default value used for SUMMARY metadata

Explain that if you don't set a value for the SUMMARY variable in a
recipe, it will get this default value from bitbake.conf:

  SUMMARY ?= "${PN} version ${PV}-${PR}"

Signed-off-by: Robert P. J. Day <[email protected]>

---

diff --git a/documentation/ref-manual/variables.rst b/documentation/ref-manual/variables.rst
index df3666f1c..bb142dc4b 100644
--- a/documentation/ref-manual/variables.rst
+++ b/documentation/ref-manual/variables.rst
@@ -10350,6 +10350,11 @@ system and gives an overview of their function and contents.
       :term:`DESCRIPTION` variable if :term:`DESCRIPTION` is
       not set in the recipe.

+      If you don't set this variable in your recipe file, it will contain
+      the default value as set in the ``bitbake.conf`` file::
+
+         SUMMARY ?= "${PN} version ${PV}-${PR}"
+
    :term:`SVNDIR`
       The directory in which files checked out of a Subversion system are
       stored.