Explicitly document the hidelines key.

Author: ehuss

guide/src/format/configuration/renderers.md

@@ -234,6 +234,9 @@ The `[output.html.code]` table provides options for controlling code blocks.
 hidelines = { python = "~" }
 ```
 
+- **hidelines:** A table that defines how [hidden code lines](../mdbook.md#hiding-code-lines) work for each language.
+  The key is the language and the value is a string that will cause code lines starting with that prefix to be hidden.
+
 ### `[output.html.search]`
 
 The `[output.html.search]` table provides options for controlling the built-in text [search].