Improve description of Python return serialization

[adamchainz]

Some improvements on exactly how return values are JSON serialized and what can happen in error cases.

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[mwunderl]

Thanks for your feedback. This may have been an issue with earlier Python runtimes.

I checked 3.6 and 3.7 with the following code-

def lambda_handler(event, context):
    f = open("/tmp/test",'wb')
    return f

I get the serialization error but no indication that the runtime crashed.
Merging just the note about serialization method.

[adamchainz]

Yes maybe it has been dealt with since... nearly a year since I submitted the PR :)