[ENH] Update the Design Details doc with our updated API.

[8veStory]

Is your feature request related to a problem? Please describe.
Depends on completion of API (#5).

Describe the solution you'd like
The following requirements are lifted from the spec:

• "Describe final architecture of your API"
• "Justify the choice of implementation"
• "Challenges addressed and shortcomings"
• "Update management information if necessary."