Include function return names in docs and Inspection of TxData

[alisinabh]

Expected results

iex(1)> Ethers.Contracts.ERC20.name()
#Ethers.TxData<function name() view returns (string name)>

iex(2)> h Ethers.Contracts.ERC20.name

                                   def name()

  @spec name() :: Ethers.TxData.t()

Executes name() on the contract.

This function should only be called for result and never in a transaction on
its own. (Use Ethers.call/2)

State mutability: view

## Return Types (when called with `Ethers.call/2`)

  • name: `:string`

Depends on poanetwork/ex_abi#151