drm/display: hdmi: Create documentation section

We have had documentation for the public functions in the HDMI helpers,
but those were never referenced anywhere and thus not compiled as part
of the doc.

Let's add a section.

Reviewed-by: Dmitry Baryshkov <lumag@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20250312-drm-hdmi-state-docs-v2-1-6352a5d68d5b@kernel.org
Signed-off-by: Maxime Ripard <mripard@kernel.org>

Documentation/gpu/drm-kms-helpers.rst

@@ -233,6 +233,21 @@ Panel Self Refresh Helper Reference
 .. kernel-doc:: drivers/gpu/drm/drm_self_refresh_helper.c
    :export:
 
+HDMI Atomic State Helpers
+=========================
+
+Overview
+--------
+
+.. kernel-doc:: drivers/gpu/drm/display/drm_hdmi_state_helper.c
+   :doc: hdmi helpers
+
+Functions Reference
+-------------------
+
+.. kernel-doc:: drivers/gpu/drm/display/drm_hdmi_state_helper.c
+   :export:
+
 HDCP Helper Functions Reference
 ===============================
 

drivers/gpu/drm/display/drm_hdmi_state_helper.c

@@ -9,6 +9,27 @@
 #include <drm/display/drm_hdmi_helper.h>
 #include <drm/display/drm_hdmi_state_helper.h>
 
+/**
+ * DOC: hdmi helpers
+ *
+ * These functions contain an implementation of the HDMI specification
+ * in the form of KMS helpers.
+ *
+ * It contains TMDS character rate computation, automatic selection of
+ * output formats, infoframes generation, etc.
+ *
+ * Testing
+ * ~~~~~~~
+ *
+ * The helpers have unit testing and can be tested using kunit with:
+ *
+ * .. code-block:: bash
+ *
+ *	$ ./tools/testing/kunit/kunit.py run \
+ *		--kunitconfig=drivers/gpu/drm/tests \
+ *		drm_atomic_helper_connector_hdmi_*
+ */
+
 /**
  * __drm_atomic_helper_connector_hdmi_reset() - Initializes all HDMI @drm_connector_state resources
  * @connector: DRM connector