recommend/require registry entries to have URIs

[dret]

when defining and maintaining a registry, it can be useful for the registry policy to recommend or require that registered values must have linkable URIs. in the simplest case, these for example can be section numbers in specs (such as RFCs).
registry maintainers may want to encourage spec writers to make sure that these URIs exist. for example, in many RFCs that register values in IANA registries, there is no well-defined section actually defining the registered value. this makes it unnecessarily hard for developers to find documentation about registered values.