Document that fee will be bumped up to min if finalFee < minFee

[EvanJRichard]

Summary

When Transaction building is done, the Transaction class checks whether the final fee is below the protocol minimum fee. If it is, the class silently bumps the fee up to the minimum. This surprises user-devs. This PR proposes to add If the final calculated fee is lower than the protocol minimum fee, the fee will be increased to match the minimum. to all main.js functions involving the fee.

Testing

Docs-only change; leaning on automated testing.

See also

Closes #224

[jasonpaulos]

It would be great to have this in the docs.