Document ReadItemAysnc item not found behavior

[jcocchi]

ReadItemAsync throws a 404 exception when an item isn't found, but this isn't clearly called out in the API reference docs. While we don't duplicate the full set of potential error codes for every API, common codes should be covered for expected, mainline scenarios.

Add a remark and example for the item not found case. Additionally, mention that ReadItemStreamAsync can be used for exception-less flows.

Additional context
There has been much discussion on this topic across several issues including #692, #1264 and others.

[bartelink]

Thanks for logging this; it'd definitely be valuable to have it addressed. (aside that I'll remove when it's gone.... typo in title!)

Please also consider resolving this in the arguably more natural and direct way (as I plead for here) by adding an actual TryGetItem API that the average developer that doesn't read the docs will naturally arrive at in the course of navigating the API set, rather than ending up doing some very indirect things, and needing people to point out concepts that don't have APIs as a navigation affordance that one can more directly refer to.