Improve consistency of examples

[sogaiu]

This PR is an attempt to improve the consistency of some of the existing examples.

Apart from a minor adjustment to correct an example, the changes basically are of three types:

1. Try to use the # -> <value> comment to describe return values only
2. Try to use the # -> error: ... (or # -> compile error: ..., etc.) comment(s) to describe errors
3. Use comments to describe printed output -- AFAICT, this is more in line with how most of the other examples do things

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Update: On a side note, I've drafted something with more details on these types of bits here.

[pepe]

LGTM

[rwtolbert]

+1