va: Update vapostproc documentation.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/merge_requests/2495>

docs/plugins/gst_plugins_cache.json

@@ -225789,6 +225789,7 @@
                 "description": "VA-API based video postprocessor",
                 "hierarchy": [
                     "GstVaPostProc",
+                    "GstVaBaseTransform",
                     "GstBaseTransform",
                     "GstElement",
                     "GstObject",
@@ -225812,6 +225813,20 @@
                         "presence": "always"
                     }
                 },
+                "properties": {
+                    "disable-passthrough": {
+                        "blurb": "Forces passing buffers through the postprocessor",
+                        "conditionally-available": false,
+                        "construct": false,
+                        "construct-only": false,
+                        "controllable": false,
+                        "default": "false",
+                        "mutable": "ready",
+                        "readable": true,
+                        "type": "gboolean",
+                        "writable": true
+                    }
+                },
                 "rank": "none"
             },
             "vavp8dec": {

sys/va/gstvafilter.c

@@ -689,6 +689,15 @@ gst_va_filter_install_properties (GstVaFilter * self, GObjectClass * klass)
             common_flags));
   }
 
+  /**
+   * GstVaPostProc:disable-passthrough:
+   *
+   * If set to %TRUE the filter will not enable passthrough mode, thus
+   * each frame will be processed. It's useful for cropping, for
+   * example.
+   *
+   * Since: 1.20
+   */
   g_object_class_install_property (klass,
       GST_VA_FILTER_PROP_DISABLE_PASSTHROUGH,
       g_param_spec_boolean ("disable-passthrough", "Disable Passthrough",