Fix bug 7648 comment in `std.stdio.File` and add bug section to `std.file.slurp`.

Issue URL: http://d.puremagic.com/issues/show_bug.cgi?id=7648
Merge pull request #1653 from denis-sh/fix-bug-7648-comments

Fix bug 7648 comment in `std.stdio.File` and add bug section to `std.file.slurp`.