[PATCH] ref-manual: Document the EXTERNALSRC_EXTENSIONS variable

[Peter Kjellerstedt <pkj@axis.com>]

EXTERNALSRC_EXTENSIONS is used to specify additional classes to inherit
when externalsrc is inherited.

Signed-off-by: Peter Kjellerstedt <peter.kjellerstedt@axis.com>
---

This of course assumes that the corresponding change to OE-Core is
accepted.

 documentation/ref-manual/variables.rst | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/documentation/ref-manual/variables.rst b/documentation/ref-manual/variables.rst
index 85f65cae35..8e7321fa20 100644
--- a/documentation/ref-manual/variables.rst
+++ b/documentation/ref-manual/variables.rst
@@ -2175,6 +2175,12 @@ system and gives an overview of their function and contents.
       ":ref:`dev-manual/common-tasks:building software from an external source`"
       section in the Yocto Project Development Tasks Manual.
 
+   :term:`EXTERNALSRC_EXTENSIONS`
+      When inheriting the :ref:`externalsrc <ref-classes-externalsrc>`
+      class, this variable contains a list of additional classes that will
+      also be inherited. This allows for extending the externalsrc
+      functionality when it is active.
+
    :term:`EXTRA_AUTORECONF`
       For recipes inheriting the :ref:`autotools <ref-classes-autotools>`
       class, you can use :term:`EXTRA_AUTORECONF` to specify extra options to