Clarify InvalidArgument details in spec.md

[akutz]

The spec's Error Scheme notes that for missing arguments a message can be added to the error_description field while invalid arguments should use the status.message field.

However, a gRPC status object does not differentiate between the two. A new status is created with an error or a message, and in both cases the error's message or the provided message are set internally to what is available via the status's message field.

I propose eliminating the confusing wording error_description and make both instances refer to the status.message field.

[jdef]

This sounds reasonable to me. @saad-ali ?

[saad-ali]

Yes, let's clean this up.