Add comment explaining exactly *why* we don't check FILE_READ_ATTRIBUTES when evaluat...

Add comment explaining exactly *why* we don't check FILE_READ_ATTRIBUTES when evaluating file/directory ACE's.

If we can access the path to this file, by
default we have FILE_READ_ATTRIBUTES from the
containing directory. See the section.
"Algorithm to Check Access to an Existing File"
in MS-FSA.pdf.
(cherry picked from commit a115a4e9799e8e5497232a149d4d927308c81a5b)