Clarify waiting semantics for UpdateWorkflow

[drewhoskins-temporal]

Is your feature request related to a problem? Please describe.
Per

sdk-go/client/client.go

Lines 575 to 586 in 8361067

	UpdateWorkflow(ctx context.Context, workflowID string, workflowRunID string, updateName string, args ...interface{}) (WorkflowUpdateHandle, error)
	// UpdateWorkflowWithOptions issues an update request to the
	// specified workflow execution and returns a handle to the update that
	// is running in in parallel with the calling thread. Errors returned
	// from the server will be exposed through the return value of
	// WorkflowUpdateHandle.Get(). Errors that occur before the
	// update is requested (e.g. if the required workflow ID field is
	// missing from the UpdateWorkflowWithOptionsRequest) are returned
	// directly from this function call.
	// NOTE: Experimental
	UpdateWorkflowWithOptions(ctx context.Context, request *UpdateWorkflowWithOptionsRequest) (WorkflowUpdateHandle, error)

it's unclear from the docstrings how to start a workflow and wait for acceptance/rejection.
I see https://github.com/temporalio/sdk-go/blob/master/internal/internal_workflow_client.go#L756 and, per the parent task, want to make sure we require WaitPolicy and don't default to anything that could be dangerous.

Describe the solution you'd like
Make sure availability tradeoff is documented clearly and user is opting in to a wait policy.

Describe alternatives you've considered

Additional context
Add any other context or screenshots about the feature request here.
Parent: temporalio/features#469