doc: internal transfer not described in manual

There is nothing about internal transfer in HomeBank manual.

1. Add "One transaction impacting 2 accounts in HomeBank with the same parameters (date, amount, category, description etc.)" in 00-lexicon.html#payment (other payment methods should have descriptions too).
2. Add new screenshot with internal transfer dialog to dlg-tran.html and describe "to account" additional parameter with note about amount registered at this account.