Mention "document outline" in public/fpdf_doc.h

In the PDF spec, document outline and bookmarks are synonyms. Mention
document outline in the public API comments, to make it more
discoverable.

Change-Id: I5ce679ef0e8da00a1d9ecc5f1cb7161cd4b383b6
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/113593
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Lei Zhang <thestig@chromium.org>
diff --git a/public/fpdf_doc.h b/public/fpdf_doc.h
index b073f4d..2dc22d9 100644
--- a/public/fpdf_doc.h
+++ b/public/fpdf_doc.h
@@ -53,6 +53,8 @@
 //
 // Returns a handle to the first child of |bookmark| or the first top-level
 // bookmark item. NULL if no child or top-level bookmark found.
+// Note that another name for the bookmarks is the document outline, as
+// described in ISO 32000-1:2008, section 12.3.3.
 FPDF_EXPORT FPDF_BOOKMARK FPDF_CALLCONV
 FPDFBookmark_GetFirstChild(FPDF_DOCUMENT document, FPDF_BOOKMARK bookmark);