Document a FPDFDOC_InitFormFillEnvironment() lifetime requirement.

Mention the FPDF_FORMFILLINFO passed in needs to outlive the handle
returned by FPDFDOC_InitFormFillEnvironment().

Bug: pdfium:1882
Change-Id: Ife9f48ba146c0163557ba335c42bd9b3d7f89bbe
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/97314
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>

public/fpdf_formfill.h

diff --git a/public/fpdf_formfill.h b/public/fpdf_formfill.h
index 5393a03..832f5b7 100644
--- a/public/fpdf_formfill.h
+++ b/public/fpdf_formfill.h
@@ -1143,11 +1143,13 @@
  *       Initialize form fill environment.
  * Parameters:
  *       document        -   Handle to document from FPDF_LoadDocument().
- *       pFormFillInfo   -   Pointer to a FPDF_FORMFILLINFO structure.
+ *       formInfo        -   Pointer to a FPDF_FORMFILLINFO structure.
  * Return Value:
  *       Handle to the form fill module, or NULL on failure.
  * Comments:
  *       This function should be called before any form fill operation.
+ *       The FPDF_FORMFILLINFO passed in via |formInfo| must remain valid until
+ *       the returned FPDF_FORMHANDLE is closed.
  */
 FPDF_EXPORT FPDF_FORMHANDLE FPDF_CALLCONV
 FPDFDOC_InitFormFillEnvironment(FPDF_DOCUMENT document,