Reformat the parts of an execution request

[benjie]

This PR adds names for each of the "pieces of information" that make up an execution request, and then shows how these values are passed into the ExecuteRequest() algorithm.

This is extracted from #976 as discussed at last night's working group. @leebyron mentioned that this change adds clarity and is a positive editorial change; splitting this change out will help keep the "extensions" PR focused as it undergoes a few iterations.

cc @michaelstaib

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