Add information about console output encoding

[Smaug123]

Summary

Adds information I failed to find earlier today, namely "what is the output encoding".

I also added information about the possibly unintuitive operation of Write{Line}(char array) when the input contains surrogates. (I claim it is indeed unintuitive, because it surprised me.) I've hedged with "may be truncated"; in my testing, Console.Write does truncate but Console.WriteLine does not, but I've tested on only one platform. I have made the rather arbitrary choice not to give the same warning on the Write(char) overload, because that one's even weirder and I'm much less confident in documenting it: whether you get re-encoded output or not depends on whether Console.Out gets flushed in between the write of the two chars in a surrogate pair or not. Of course it's implicitly illegal to write surrogates out at all, but it didn't seem to be documented anywhere obvious that this is so.

[learn-build-service-prod]

Learn Build status updates of commit fd68108:

✅ Validation status: passed

File	Status	Preview URL	Details
xml/System/Console.xml	✅Succeeded	View

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[Smaug123]

(I reordered this to put my comment first, because otherwise there's a syntactic ambiguity about whether my comment is conditioned on the "value is null" from the other comment.)