Improve documentation around usage of `LocalWorkspace.CreateAsync`

[smithrobs]

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Issue details

Slightly related to the tests for #382 - it seems LocalWorkspace.CreateAsync requires either WorkDir or ProjectSettings to be set. Otherwise, when attempting to delete the stack an unhelpful error is given as (I assume) there's no project to infer:

  Error Message:
   Pulumi.Automation.Commands.Exceptions.CommandException : code: 255
stdout: 
stderr: error: if you're using the --stack flag, pass the fully qualified name (organization/project/stack)

  Stack Trace:
     at Pulumi.Automation.Commands.LocalPulumiCommand.RunAsyncInner(IList`1 args, String workingDir, IDictionary`2 additionalEnv, Action`1 onStandardOutput, Action`1 onStandardError, String stdIn, EventLogFile eventLogFile, CancellationToken cancellationToken) in /_/sdk/Pulumi.Automation/Commands/LocalPulumiCommand.cs:line 356
   at Pulumi.Automation.Commands.LocalPulumiCommand.RunInputAsync(IList`1 args, String workingDir, IDictionary`2 additionalEnv, Action`1 onStandardOutput, Action`1 onStandardError, String stdIn, Action`1 onEngineEvent, CancellationToken cancellationToken) in /_/sdk/Pulumi.Automation/Commands/LocalPulumiCommand.cs:line 302 

• Is there an opportunity to better document this behavior?
• Is the expectation that the usage of LocalWorkspace.CreateAsync include project data outside of the ctor at some point?
• For this scenario, could the error message be made clearer?

Affected area/feature

Automation API