Ignore non doc attributes at top level genrated by ppx

CHANGES.md

@@ -12,6 +12,7 @@ Bugs fixed
   functor arguments to have different filenames (@jonludlam, #795)
 - Better memory/disk space usage when handling module alias chains (@jonludlam, #799)
 - Resolving class-type paths (ie., `val x : #c`) (@jonludlam, #???)
+- Skip top-level attributes while extracting the top comment. Fix top-comment extraction with PPX preprocessing (@jorisgio, #819)
 
 2.0.2
 -----

doc/odoc_for_authors.mld

@@ -576,10 +576,12 @@ module M : sig
   end
 ]}
 
-As an exception, [open] statements are allowed to be placed before the top-comment.
+As an exception, [open] statements and attributes are allowed to be placed before the top-comment.
 For example:
 
 {[
+[@@@ocaml.warning "-6"]
+
 (* Copyright header *)
 
 open Base

src/loader/doc_attr.ml

@@ -147,7 +147,9 @@ let extract_top_comment internal_tags ~classify parent items =
                   |> Error.raise_parser_warnings
                 in
                 (tl, ast_docs)
-            | None -> (items, []))
+            | None ->
+                let items, ast_docs = extract ~classify tl in
+                (hd :: items, ast_docs))
         | Some `Open ->
             let items, ast_docs = extract ~classify tl in
             (hd :: items, ast_docs)