Address async issues

[BillWagner]

Each fix is in a single commit. In order:

Fixes #857: produces a result vs. returns a value.

For an async method, say "produces a result value" vs. "return a value" when describing the rules for the returned "task type".

Fixes #878

Add a note that the builder pattern supports identity convertible types. I made this a note, rather than normative language. I read the existing text to allow the example in the issue (as does the roslyn implementation).

Fixes #775

Add a bullet to the list specifying that an OperationCanceledException puts the returned task in the cancelled state.

This meant adding OperationCanceledException to the types in Annex C.

I chose this fix because all known task builders conform to this behavior. It seems reasonable to standardize it.

Fixes #879

Clarify the required accessibility of any task builder type and its required members. The members of the task builder type must have declared public accessibility. The accessibility of the task builder type must match the accessibility of its corresponding task type.

Note that we will need to update this text once dotnet/roslyn#71283.

The fifth commit adds a note clarifying the meaning of the accessibility requirements for the task builder type and its corresponding task type.

[jskeet]

Mostly fine (and thanks!), a few comments/questions.

[jskeet]

LGTM again, after discussion.