On the "Sending messages" page, add a section regarding forward fees, from what balance and how they're paid in respect to different modes

[novusnota]

As suggested by @skywardboundd.

1. Forward fees are deducted from the contract's balance, not the incoming message value.
2. However, forward fees for the bounced messages are taken from its value. If there were none, the message won't bounce.
3. Describe forward fees for each mode.