Add note about Issue 7648 to `std.stdio.File`.

Issue URL: http://d.puremagic.com/issues/show_bug.cgi?id=7648
Fix `std.traits.hasElaborateAssign` for static arrays.
Merge pull request #954 from denis-sh/fix-std.traits.hasElaborateAssign

Fix `std.traits.hasElaborateAssign`
Merge pull request #1238 from denis-sh/add-note-about-Issue-7648

Add note about Issue 7648 to `std.stdio.File`.