Improve documentation for decoding a single key vs multiple keys

[CJ42]

Summary

The documentation does not make it clear that it is possible to use ERC725.decodeData(...) to decode both:

• a single data key, passing an object as parameter.
• multiple data keys, passing an array of objects as parameter.

See examples below

This can be useful to know, as encodeData only works with objects right now, so array destructuring can be cumbersome at times.

https://github.com/ERC725Alliance/erc725.js/blob/develop/docs/methods.md#single-key-example