Merge state v2 forward compatibility issues, and a proposal for v3

[timeless]

Pierre-Yves David wrote:
> The way we've been handling (5) is usually to point to a wiki page that
> explain the use each option and how to get compatible. This is used at least
> for "requires" file and "bundle format and capabilities".
>
> I think this will be more flexible and good enough.

yep, that's more or less the best you can do, since it doesn't require
a client update, and can provide an easy way for localizers to hook in
an explanation if they want (i.e. they could have a translated page
for that page, a la Wikipedia's "in other languages" section)