Document unusual wl_registry.bind new_id behavior

When wayland-scanner encounters a new_id field with no corresponding
interface name defined, instead of emitting a function whose signature
lines up with the usual case (a uint32_t ID), it adds the interface name
as a string and the version number so that the interface can be
identified from the protcol message.

Without docs, this was previously left for the interprid wire protocol
implementor (e.g. me an hour ago) to discover when Wayland clients send
them apparently bogus messages.

I would have preferred if a different primitive type were used here
(e.g. typed_new_id) to reflect the fact that the wire protocol is
different, but I felt it unwise to add a new primitive to wayland.xml in
$current_year.
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