Improve mapping documentation

Merged Rich requested to merge artfulrobot/advimport:art-improve-mapping-documentation into master

I found the documentation for how get mapping insufficient to tell me what to do, hope this is a helpful improvement.

As an aside: I'm not sure what the value of having a field key is, when this MUST be the same as the mapping's key. Is there a case for this or could it be simplified?

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