gtk/css-provider: Add back array length annotation

This partially reverts commit 93a875bf20.

Removing this annotation broke bindings, which now treat the
length of the array as a standalone parameter.

This broke Spiel, and probably other non-C apps that rely on
GtkCssProvider through bindings.

gtk/gtkcssprovider.c

@@ -1089,7 +1089,7 @@ gtk_css_provider_load_internal (GtkCssProvider *self,
 /**
  * gtk_css_provider_load_from_data:
  * @css_provider: a `GtkCssProvider`
- * @data: CSS data to be parsed
+ * @data: (array length=length): CSS data loaded in memory
  * @length: the length of @data in bytes, or -1 for NUL terminated strings
  *
  * Loads @data into @css_provider.