New docs section for canceled-context in workflows, other minor cleanups

[Groxx]

Canceled-context documentation is currently strewn about in individual function documentation, though it's a fairly important concept that needs to be understood.

In particular, users sometimes are under the impression that "all things which accept a context will error", which can lead to nasty surprises like selector.Select(ctx)'s blocking behavior (causing a deadlocked workflow). Selector's docs have been improved in #1131, and this will hopefully make its behavior easier to learn before it is needed.

And, since I noticed some small other things while writing this chunk of docs, I've rolled those changes into here as well.

[coveralls]

Pull Request Test Coverage Report for Build fcc0307a-17f1-4226-a3a7-ff6129226ebd

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 63.29%

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Totals
Change from base Build 6f8d6cac-3789-4a94-b2a6-81a36ff95f17:	0.02%
Covered Lines:	12234
Relevant Lines:	19330

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💛 - Coveralls