Add note for FPDFBookmark_GetNextSibling() circular references handling.

Mention that the caller is responsible for dealing with it.

Bug: pdfium:1759
Change-Id: I508caced7fd2d7078d40cb259c8f03ebc09fdbd4
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/87990
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
diff --git a/public/fpdf_doc.h b/public/fpdf_doc.h
index 350b964..df453ad 100644
--- a/public/fpdf_doc.h
+++ b/public/fpdf_doc.h
@@ -74,6 +74,9 @@
 //
 // Returns a handle to the next sibling of |bookmark|, or NULL if this is the
 // last bookmark at this level.
+//
+// Note that the caller is responsible for handling circular bookmark
+// references, as may arise from malformed documents.
 FPDF_EXPORT FPDF_BOOKMARK FPDF_CALLCONV
 FPDFBookmark_GetNextSibling(FPDF_DOCUMENT document, FPDF_BOOKMARK bookmark);