Tweak "producer as parent" config option

[unflxw]

Some minor tweaks to the changes introduced in #11. See #10 for context.

Rename the config option to useProducerSpanAsConsumerParent (that is, s/Context/Span) to use language that is more familiar to casual OpenTelemetry users.

Rewrite the config option description to describe what happens with and without the config option.

Rearrange the instrumentation to move logic branches out of the span creation function.

[ping @Steffen911, in case you're using a pre-release version, note the config option name has been changed!]

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