doc(context fun): fix example, add some more explanations

[robstoll]

Note that the used syntax in the second example actually does not work (I have left a TODO)

[robstoll]

I realised that given new Table doesn't compile. Intentionally?

[robstoll]

I guess I misinterpreted the syntax and given T is actually a special shorthand for given T = new T() and not a creator application. Likewise class A(i : Int); given A(1) is short for given A = new A(1) correct?

if so then I have to update this PR and in this case I would suggest that it is mentioned in http://dotty.epfl.ch/docs/reference/contextual/givens.html

I would do the necessary with an additional commit.

Thoughts?

[odersky]

Thanks for the fixes, this is great!

[robstoll]

@odersky I removed the TODO as the syntax which previously did not work in scastie compiles perfectly fine in master (and now also on scastie):
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