Improved PE class comments

PEImage and PeImageReader are similar classes with slightly different
behaviors. This change updates the comments to clarify how they differ.

Change-Id: Id09868324b93d5a44731f778fb3f7684ca77736f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2336303Reviewed-by: Will Harris <wfh@chromium.org>
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>
Auto-Submit: Bruce Dawson <brucedawson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#795188}

base/win/pe_image.h

@@ -26,7 +26,12 @@ namespace win {
 
 // This class is a wrapper for the Portable Executable File Format (PE).
 // Its main purpose is to provide an easy way to work with imports and exports
-// from a file, mapped in memory as image.
+// from a file, mapped in memory as image. A PEImage object is constructed from
+// a loaded PE file by passing the HMODULE to the constructor. Loading a PE file
+// as an image will execute code and should only be done with trusted images.
+// Parsing of untrusted PE files is better done with PeImageReader.
+// PEImage can only parse PE files that match the bitness of the process.
+// See also PEImageAsData.
 class PEImage {
  public:
   // Callback to enumerate sections.

base/win/pe_image_reader.h

@@ -18,7 +18,11 @@ namespace base {
 namespace win {
 
 // Parses headers and various data from a PE image. This parser is safe for use
-// on untrusted data.
+// on untrusted data and works on PE files with different bitness from the
+// current process. The PeImageReader is initialized after construction by
+// passing the address and size of a PE file that has been read into memory -
+// not loaded by the OS as an image. Parsing of a PE file that has been loaded
+// as an image can be done with PEImage.
 class BASE_EXPORT PeImageReader {
  public:
   enum WordSize {