diff -u b/core/themes/classy/templates/block--search-form-block.html.twig b/core/themes/classy/templates/block--search-form-block.html.twig
--- b/core/themes/classy/templates/block--search-form-block.html.twig
+++ b/core/themes/classy/templates/block--search-form-block.html.twig
@@ -6,13 +6,13 @@
  * Available variables:
  * - plugin_id: The ID of the block implementation.
  * - label: The configured label of the block if visible.
- * - configuration: A list of the block's configuration values.
+ * - configuration: A list of the block's configuration values, including:
  *   - label: The configured label for the block.
  *   - label_display: The display settings for the label.
  *   - module: The module that provided this block plugin.
  *   - cache: The cache settings.
  *   - Block plugin specific settings will also be stored here.
- * - block - The full block entity.
+ * - block - The full block entity, including:
  *   - label_hidden: The hidden block title value if the block was
  *     configured to hide the title ('label' is empty in this case).
  *   - module: The module that generated the block.
@@ -20,7 +20,7 @@
  *   - region: The block region embedding the current block.
  * - content: The content of this block.
  * - attributes: A list HTML attributes populated by modules, intended to
- *   be added to the main container tag of this template.
+ *   be added to the main container tag of this template. Includes:
  *   - id: A valid HTML ID and guaranteed unique.
  * - title_attributes: Same as attributes, except applied to the main title
  *   tag that appears in the template.