devguides: fixes to the get_address descriptions

[dimalinux]

Fixes incorrect descriptions of the encoding used for addresses. Let's the user know that the primary address is also returned. Includes information on the default behavior when the address_index field is omitted. (Note that address_index should have been named address_indices, as it is in the outputs of create_address, but that's not something the documentation can fix.)

[netlify[bot]]

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