xdg-activation: rewrite and move description of token forwarding

After a requesting client receives a new token, the client usually forwards the
token string to another client by a different process, which then uses the
token in an activate request. For that the token string must be transferred to
the other process.

Two default ways of doing that were described in the done event, but the
description had some issues and it makes more sense to describe them in the
protocol description itself, which talks about the protocol in a more general
way. Therefore rewrite the paragraphs about token forwarding between clients
and place them in the protocol description.

Signed-off-by: Roman Gilg <subdiff@gmail.com>