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

Issue URL: http://d.puremagic.com/issues/show_bug.cgi?id=7648
Fix issue in `std.json` when `null` is returned instead of empty string/array.
DDoc -std.stdio: exceptions thrown by C-like API.

Updated documentation for all the C-like member functions \
(and the File ctor) to specify exactly what gets thrown when. 

(note: Exceptions in std.stdio are a mess and should be made 
 more consistent. Also, it seems some member functions that 
 should check if the file is opened do not, but I don't know
 if that might be intentional (as the C functions should fail 
 anyway, resulting in a throw))
Merge pull request #1239 from denis-sh/std.json-improvements-2

Fix issue in `std.json` when `null` is returned instead of empty string/array.
Merge pull request #1252 from kiith-sa/patch-2

DDoc -std.stdio: exceptions thrown by C-like API.
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`.