notes: move the documentation to the struct

Its better to document the struct members directly instead of on a
function that takes a pointer to the struct. This will also make it
easier to update the documentation in the future.

Make adjustments for this new context. Also drop “may contain” since we
don’t need to emphasize that a list could be empty.

Suggested-by: Jeff King <peff@peff.net>
Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

notes.h

@@ -256,7 +256,17 @@ void free_notes(struct notes_tree *t);
 struct string_list;
 
 struct display_notes_opt {
+	/*
+	 * Less than `0` is "unset", which means that the default notes
+	 * are shown iff no other notes are given. Otherwise,
+	 * treat it like a boolean.
+	 */
 	int use_default_notes;
+
+	/*
+	 * A list of globs (in the same style as notes.displayRef) where
+	 * notes should be loaded from.
+	 */
 	struct string_list extra_notes_refs;
 };
 
@@ -283,15 +293,7 @@ void disable_display_notes(struct display_notes_opt *opt, int *show_notes);
 /*
  * Load the notes machinery for displaying several notes trees.
  *
- * If 'opt' is not NULL, then it specifies additional settings for the
- * displaying:
- *
- * - use_default_notes: less than `0` is "unset", which means that the
- *   default notes are shown iff no other notes are given. Otherwise,
- *   treat it like a boolean.
- *
- * - extra_notes_refs may contain a list of globs (in the same style
- *   as notes.displayRef) where notes should be loaded from.
+ * 'opt' may be NULL.
  */
 void load_display_notes(struct display_notes_opt *opt);