Corrected/clarified invalid Simple Send conditions

Correction made: it's not valid to send more coins than are owned and available.

Updated the doc version to 0.4.5.4

Is the following excerpt accurate for how Sell Offers are validated:

"If the amount offered for sale exceeds the sending address's available balance (the amount not reserved, committed or in escrow), this indicates the user is offering to sell all coins that are available at the time this sell offer is published."