subscriber: add docs on registry span ID generation

[hawkw]

Motivation

Currently, tracing-subscriber's Registry type doesn't document how
span IDs are generated, or the uniqueness guarantees for span IDs. This
can cause confusion for users trying to implement Subscribers and
other code that interacts with the Registry's generated span IDs.

Solution

This branch adds a new documentation section to the Registry docs
describing how IDs are generated and when they are guaranteed to be
unique. In particular, the new section explicitly states that the
registry's IDs will not uniquely identify a span historically, because
IDs may be reused when a span has closed, and that the registry's
tracing span IDs should not be used as span IDs in a distributed
tracing system.

While I was working on these docs, I also fixed a link on adding
subscribers to a registry that went to a specific method on
FmtSubscriber, rather than to the more general docs on how
subscribers are composed with collectors.

Thanks to @Folyd for suggesting this docs improvement!