Add comment about local encoding in cpdfsdk_formfillenvironment.cpp

Converting strings to/from DefANSI is almost always a mistake, so
document two cases where this is absolutely required.

Change-Id: Iaec6029f22b5f37a03d3c31cf1197bc8515701e3
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/93791
Reviewed-by: Lei Zhang <thestig@chromium.org>
Auto-Submit: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>

fpdfsdk/cpdfsdk_formfillenvironment.cpp

diff --git a/fpdfsdk/cpdfsdk_formfillenvironment.cpp b/fpdfsdk/cpdfsdk_formfillenvironment.cpp
index 8e9eac9..b73f971 100644
--- a/fpdfsdk/cpdfsdk_formfillenvironment.cpp
+++ b/fpdfsdk/cpdfsdk_formfillenvironment.cpp
@@ -241,6 +241,8 @@
 
   // Don't include trailing NUL.
   pBuff.resize(nActualLen - 1);
+
+  // Use FromDefANSI() per "local encoding" comment in fpdf_formfill.h.
   return WideString::FromDefANSI(ByteStringView(pBuff));
 }
 
@@ -314,6 +316,8 @@
 
   // Don't include trailing NUL.
   pBuff.resize(nActualLen - 1);
+
+  // Use FromDefANSI() per "local encoding" comment in fpdf_formfill.h.
   return WideString::FromDefANSI(ByteStringView(pBuff));
 }