Clarify text "as needed"

[EskoDijk]

5.2.2 has currently:

A stub router MUST allocate a single ULA prefix for use in providing on-link prefixes to the stub network and the infrastructure network, as needed.

The "as needed" part here is not so clear and needs to be detailed. E.g.

• only if an on-link prefix on the AIL is not present, the stub router provides one from its own ULA prefix
• only if the stub network does not have an on-link prefix or on-mesh prefix (whatever applies), then it creates one from its own ULA prefix.
• sizes of the involved prefixes? Better to either specify sizes or say it's up to implementation.
• state some randomness requirements - although for the ULA Global ID it may be clear that it must be randomized; we also have Subnet ID to have requirements for (or not). In any case state these assumptions.

[EskoDijk]

This can be resolved easily by a pointer to other parts of the draft ("as defined in Section N"). Because there are still some pending PRs that impact the text here and add the details, this issue can be postponed until after these PRs are merged.