Update comments about Backend interface implementation, add implementation assertion

[SarahFrench]

While working through the docs at docs/architecture.md
I realised that the backend package has some old comments that predate when the backend.Enhanced interface was replaced in #34818.

This PR:

• Updates old comments
• Adds a missing check that the Local backend implements the new interface backendrun.OperationsBackend

Target Release

N/A

Draft CHANGELOG entry

N/A

[SarahFrench]

To be fair, this was already checked in a test:

terraform/internal/backend/local/backend_test.go

Lines 20 to 24 in 0d6c1d3

	func TestLocal_impl(t *testing.T) {
	var _ backendrun.OperationsBackend = New()
	var _ backendrun.Local = New()
	var _ backendrun.CLI = New()
	}

But this assertion will run at compile time

[github-actions]

Reminder for the merging maintainer: if this is a user-visible change, please update the changelog on the appropriate release branch.

[github-actions]

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