Add a note about FPDFText_GetText() behavior.

Explain that the returned results contain characters outside the
cropbox, and suggest some APIs to call to determine if that is the case.

Bug: pdfium:1842
Change-Id: I2ac79668184f5313a14a3d3d6f3f4a8eda06d678
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/95251
Reviewed-by: Nigi <nigi@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>

public/fpdf_text.h

diff --git a/public/fpdf_text.h b/public/fpdf_text.h
index 6d4a020..65604d8 100644
--- a/public/fpdf_text.h
+++ b/public/fpdf_text.h
@@ -341,6 +341,10 @@
 //          trailing terminator.
 // Comments:
 //          This function ignores characters without unicode information.
+//          It returns all characters on the page, even those that are not
+//          visible when the page has a cropbox. To filter out the characters
+//          outside of the cropbox, use FPDF_GetPageBoundingBox() and
+//          FPDFText_GetCharBox().
 //
 FPDF_EXPORT int FPDF_CALLCONV FPDFText_GetText(FPDF_TEXTPAGE text_page,
                                                int start_index,