Better abstract/introduction?

[plehegar]

From
https://lists.w3.org/Archives/Public/public-html-media/2016Jan/0000.html
[[
We would request greater use of prose to describe what the API
is doing. This is needed both to help the reader understand what is
being specified, but also to test whether the proposed API actually
meets its intended purpose.
]]

[wolenetz]

This looks like V1NonBlocking per step 1 of our triage process.

[jdsmith3000]

I added a very simple introduction description, just above the diagram. I did not attempt to add a detailed text description of the diagram itself.

[wolenetz]

See my comment at #134 (comment), where I note that I also landed 39795b6 as part of the fix for this (to retain the normative reference to HTML51 which @jdsmith3000's PR had accidentally removed).