fungible traits: improve api docs

[liamaharon]

          > > Just want to add here, I do not think there is enough education around how to properly work with the [`fungible`](https://paritytech.github.io/substrate/master/frame_support/traits/tokens/fungible/index.html) traits - it can be overwhelming for a newcomer (and at times confusing).

It would be nice to have a playground or have examples that showcases how to use the many different fungible traits.

#225 might help with this. @liamaharon based on the above, would be great if you try and improve the documentation of these APIs, probably add more examples and so on as well.

Sure. I'll create a new issue to track improving the docs, and work on those as I go with the tests. I've tried to include descriptive inline comments in all of the tests so they should be good candidates to include as examples with the help of docify.

_Originally posted by @liamaharon in #186

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Documentation for individual methods I think is already pretty good. I feel like part of what can make it overwhelming for new users is they're presented with a bunch of new concepts all at once, with little structure around how they all relate. The traits could probably benefit from top-level docs introducing readers to fungible/s one concept at a time, assuming no prior knowledge.

Note so I don't forget: request review from @brunopgalvao on the eventual MR for this issue.

items to not forget (non-exhaustive)

• existential deposit and reaping behavior