More documentations on srml support traits including Currency, ChargeFee, TransferAsset

[xlc]

Good to see comprehensive documentations have been written for all the modules but I think documentations for srml support traits are lacking.

SRML developers (and anyone developing reusable modules) should avoid tight coupling between modules and this is achieved by depend on traits instead of modules directly.

Developers doesn't really need to know how to use Balances::free_balance because they should be using Currency::free_balance instead. Another example is Fees::charge_fee and ChargeFee::charge_fee.

[xlc]

Close as the issue belongs to substrate-developer-hub