[booru] add an option to extract notes (only gelbooru for now) (#1457)

* [booru] add an option to extract notes (currently implemented only for gelbooru)

* appease linter

* [gelbooru] rename "text" to "body" in note extraction

* add a code comment about reusing return value of _extended_tags

docs/configuration.rst

@@ -1747,6 +1747,16 @@ Description
 
     Note: This requires 1 additional HTTP request for each post.
 
+extractor.[booru].notes
+----------------------
+Type
+    ``bool``
+Default
+    ``false``
+Description
+    Extract overlay notes (position and text).
+
+    Note: This requires 1 additional HTTP request for each post.
 
 extractor.[manga-extractor].chapter-reverse
 -------------------------------------------