Document the bound domain in provider initialization and domain-scoped providers

[jonathannorris]

The spec now supplies the bound domain to a provider's initialize function, and adds an opt-in domain-scoped provider declaration that the API enforces by binding such a provider to at most one domain. The conceptual docs should cover this so provider authors and users understand it.

What needs to happen

• Document that initialize can receive the bound domain, and when a provider would use it (e.g. scoping a persistent cache)
• Document the domain-scoped provider concept: what declaring it means, and that the API binds such a provider to at most one domain
• Fit this into the existing provider/domain and provider-lifecycle docs rather than a standalone page

Related

• docs: add provider lifecycle guidance #1387 (provider lifecycle guidance), [DOC] add init/shutdown documentation #119 (init/shutdown docs)

Links

• Parent issue: Supply the bound domain to provider initialization across SDKs spec#403
• Spec sections: 1.1 Flag evaluation, 2.4 Initialization