[wwwdocs] Add a note about in-class initialization of static data member

[Marek Polacek]

On Thu, Feb 11, 2016 at 01:36:49PM -0700, Martin Sebor wrote:
> >>The flexible array addition looks great to me. Thank you!
> >
> >Great.  I'll commit the patch.
> 
> Actually, there is one other thing that might be wort mentioning
> about flexible array members.
> 
> The type and mangling of flexible array members has changed.  While
> in GCC 5 and prior the type of a flexible array member is an array
> of zero elements (a GCC extension), in 6 it is that of an array of
> an unspecified bound (i.e., T[] as opposed to T[0]).  This is
> a silent ABI change with no -fabi-version/-Wabi option.

Aha.  I think you're better-suited to document this than I am.

	Marek