Make startUpdate users aware that it's synchronous w/ worker

[drewhoskins-temporal]

Is your feature request related to a problem? Please describe.
A clear and concise description of what the problem is. Ex. I'm always frustrated when [...]
temporalio/features#469

Describe the solution you'd like
A clear and concise description of what you want to happen.
Related to

sdk-java/temporal-sdk/src/main/java/io/temporal/client/WorkflowStub.java

Lines 94 to 106 in ed211fa

	/**
	* Asynchronously update a workflow execution by invoking its update handler and returning a
	* handle to the update request. Usually a update handler is a method annotated with {@link
	* io.temporal.workflow.UpdateMethod}.
	*
	* @param updateName name of the update handler. Usually it is a method name.
	* @param resultClass class of the update return value
	* @param <R> type of the update return value
	* @param args update method arguments
	* @return update handle that can be used to get the result of the update.
	*/
	@Experimental
	<R> UpdateHandle<R> startUpdate(String updateName, Class<R> resultClass, Object... args);

Developers should be informed about and opt in to behavior that startWorkflow waits for acceptance

Describe alternatives you've considered

Additional context