Update documentation for FPDFBitmap_GetBuffer()

Mention using FPDFBitmap_GetFormat() to find out the data format,
instead of assuming it is always BGRA.

Change-Id: I3cb975c42b6a670b426eba7fee6f0751dac38948
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/104892
Commit-Queue: Nigi <nigi@chromium.org>
Auto-Submit: Lei Zhang <thestig@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Nigi <nigi@chromium.org>
diff --git a/public/fpdfview.h b/public/fpdfview.h
index 32db6d1..206bfe2 100644
--- a/public/fpdfview.h
+++ b/public/fpdfview.h
@@ -1173,8 +1173,7 @@
 //          then manipulate any color and/or alpha values for any pixels in the
 //          bitmap.
 //
-//          The data is in BGRA format. Where the A maybe unused if alpha was
-//          not specified.
+//          Use FPDFBitmap_GetFormat() to find out the format of the data.
 FPDF_EXPORT void* FPDF_CALLCONV FPDFBitmap_GetBuffer(FPDF_BITMAP bitmap);
 
 // Function: FPDFBitmap_GetWidth