Improve ExecutionContext implicitNotFound and docs

It is not good practice to import a specific ExecutionContext
all over the place; we shouldn't recommend that. People should
allow callers to specify the context in most cases and only
import the context in some central location in their code.

While we are at it, add some more comprehensive docs to
ExecutionContext which will hopefully give people enough understanding
to make decisions about it.
Merge pull request #3432 from havocp/patch-1

Improve ExecutionContext implicitNotFound and docs