Add an informative note on design constraint for speech events

[jugglinmike]

In gh-90, we agreed that fundamental constraints make it impossible for this protocol to correlate "user intent" commands and any AT responses (e.g. speech) that they induce. However, an application developer may not be aware of the realities which inform these constrains, making the spec's current design seem arbitrarily cumbersome. Extend the spec with a succinct non-normative note that explains the rationale.