[03/34] ref-manual: expand documentation on image-buildinfo class

diff --git a/documentation/ref-manual/classes.rst b/documentation/ref-manual/classes.rst
index 5e49613..cc0d212 100644
--- a/documentation/ref-manual/classes.rst
+++ b/documentation/ref-manual/classes.rst
@@ -881,8 +881,22 @@  Yocto Project Overview and Concepts Manual.
 ``image-buildinfo.bbclass``
 ===========================
 
-The ``image-buildinfo`` class writes information to the target
-filesystem on ``/etc/build``.
+The ``image-buildinfo`` class writes a plain text file containing
+build information to the target filesystem at ``${sysconfdir}/buildinfo``
+by default (as specified by :term:`IMAGE_BUILDINFO_FILE`.
+This can be useful for manually determining the origin of any given
+image. It writes out two sections:
+
+1. `Build Configuration`: a list of variables and their values (specified
+   by :term:`IMAGE_BUILDINFO_VARS`, which defaults to :term:`DISTRO` and
+   :term:`DISTRO_VERSION`)
+
+2. `Layer Revisions`: the revisions of all of the layers used in the
+   build.
+
+Additionally, when building an SDK it will write the same contents
+to ``/buildinfo`` by default (as specified by
+:term:`SDK_BUILDINFO_FILE`).
 
 .. _ref-classes-image_types:
 
diff --git a/documentation/ref-manual/variables.rst b/documentation/ref-manual/variables.rst
index 8ea9834..8b0331c 100644
--- a/documentation/ref-manual/variables.rst
+++ b/documentation/ref-manual/variables.rst
@@ -3053,6 +3053,17 @@  system and gives an overview of their function and contents.
       material for Wic is located in the
       ":doc:`/ref-manual/kickstart`" chapter.
 
+   :term:`IMAGE_BUILDINFO_FILE`
+      When using the :ref:`image-buildinfo <ref-classes-image-buildinfo>` class,
+      specifies the file in the image to write the build information into. The
+      default value is "``${sysconfdir}/buildinfo``".
+
+   :term:`IMAGE_BUILDINFO_VARS`
+      When using the :ref:`image-buildinfo <ref-classes-image-buildinfo>` class,
+      specifies the list of variables to include in the `Build Configuration`
+      section of the output file (as a space-separated list). Defaults to
+      ":term:`DISTRO` :term:`DISTRO_VERSION`".
+
    :term:`IMAGE_CLASSES`
       A list of classes that all images should inherit. You typically use
       this variable to specify the list of classes that register the
@@ -6627,6 +6638,11 @@  system and gives an overview of their function and contents.
       The target architecture for the SDK. Typically, you do not directly
       set this variable. Instead, use :term:`SDKMACHINE`.
 
+   :term:`SDK_BUILDINFO_FILE`
+      When using the :ref:`image-buildinfo <ref-classes-image-buildinfo>` class,
+      specifies the file in the SDK to write the build information into. The
+      default value is "``/buildinfo``".
+
    :term:`SDK_CUSTOM_TEMPLATECONF`
       When building the extensible SDK, if :term:`SDK_CUSTOM_TEMPLATECONF` is set to
       "1" and a ``conf/templateconf.cfg`` file exists in the build directory